The "River Tank": A Versatile Tool for Teaching Science

Christopher H. Randall
Center for the Enhancement of Science and Math Education
Northeastern University, Boston, MA 02155

Imagine your students making observations and direct measurements of a stream ecosystem, gaining insight into its dynamics, and developing an appreciation of its complexity without ever leaving the classroom. The River Tank System is being used to conduct engaging, classroom-based "field studies" in biology and chemistry classes throughout the country. Finn Strong, the inventor of River Tank, originally envisioned this habitat as being popular with aquarium enthusiasts and in home and office displays. Not surprisingly, however, teachers have given the tank an enthusiastic reception, and it has become a versatile and valuable teaching tool in elementary, middle, and high school.

A Complete Habitat

The River Tank combines elements of an aquarium, a terrarium, and an animal exhibit to produce a dynamic habitat where plants, animals, and microorganisms interact in ecological balance. Functionally, what distinguishes the River Tank from a traditional aquarium is the flowing water. A pump raises the water to an upper pool whose surface is 20 centimeters above the surface of the bottom pool. The water flows downstream through a series of pools, waterfalls, and rapids before being recirculated (Fig. 1). The result is a model of a stream with shallow and deep pools, slow and swift currents, rapids, eddies, and waterfalls. Fish swim freely from pool to pool, and students can observe how fish use the currents to help propel themselves up the rapids and waterfalls.

Unlike a traditional aquarium, the River Tank also serves as a terrarium with cavities and ledges providing places to grow a variety of plants, which utilize fish waste and help keep the tank in balance by removing nutrients from the water. In a classroom version of the water cycle, the plants on the upper ledges are watered by drops of condensed vapor. Reflecting the diversity of life in and around a stream, lizards, frogs, turtles, and insects also use the banks and ledges as living areas.

Finally, the River Tank is designed to promote robust bacterial growth. Bacteria grow on practically every available submerged surface in the tank, and the large, gravel-filled cavity in the center of the tank (Fig. 2) vastly increases the surface area for their growth. Additionally, a series of small holes in the wall of the cavity allows oxygenated, nutrient-rich water to seep slowly through the gravel, which promotes an ideal, aerobic environment for bacteria. By supporting such large, healthy bacterial populations, the River Tank has many times the biologic filtering capacity of a traditional aquarium.

A Versatile Tool

There are five environments in the River Tank - underwater in the slow currents, underwater in the fast currents, a bog, the water's edge, and high above the water on the ledges. A class might consider how many different types of organisms can live successfully in the tank and populate each of these environments. Turtles, leeches, crayfish, snails, (Fig. 3) worms, fish, salamanders, American chameleons (Fig. 4) newts, insects, bacteria, algae, higher plants, and many protists all can be successfully raised in the River Tank, but questions remain as to which ones can coexist, and for how long, before being eaten by another inhabitant.

Likewise, by describing the characteristics of the plants that grow in these different environments, students gain an understanding of the concept of niches. They can predict which plants will thrive and can test their predictions by establishing a viable plant community in the tank.

Establishing the fish collection poses a similar challenge for students. For example, the 20-gallon tank holds about 12 gallons of water and can support twelve to sixteen fish. A class might consider the following questions when determining which fish to include:

  1. Which species are considered community fish, and which are considered aggressive?
  2. Do all fish do well in a moving-water environment?
  3. How many herbivores and scavengers does a tank this size require?
  4. Do streams have bottom, mid-level, and surface feeders, as ponds do?

Once the fish colony is established, students can investigate additional questions:

  1. Do fish orient themselves in predictable ways?
  2. Does the water level (and thus waterfall size) affect fish migration between the pools?
  3. Are certain species territorial?
  4. Does crowding affect certain species more than others?
  5. Since territorial fish often establish themselves near caves, does creating caves and overhangs change the behavior of certain fish?

Modeling an Ecosystem

By determining the roles that plants, animals, and microorganisms play in the River Tank system, students gain insight into the relationship between producers, consumers, scavengers, and decomposers. Furthermore, by studying its chemistry, students realize how utterly dependent the biotic community is on the abiotic components of the system (i.e., light, temperature, inorganic compounds, and oxygen and carbon dioxide levels).

A class could synthesize its understanding of all that is required to create a balanced ecosystem by modeling a particular type of stream. Since aquarium fish come from all over the world, a class could set up a tank to represent a particular continent or biome and choose plants and animals accordingly. Students also could contrast various steam ecosystems by setting up multiple tanks or by making picture essays or oral presentations.

Such projects would raise important questions about the extent to which one can accurately model a natural ecosystem, and about how the River Tank differs from such a natural system. With such questions answered, the "River Tank" serves as an ideal introduction for a field trip to a local stream or river, enabling a class to maximize the value of its visit.

The questions raised by establishing a balanced ecosystem in the tank can be the basis for sparking class discussion and for generating research projects. By reflecting on what they learned, students become sensitive to the effects of destroying part of an ecosystem or of removing a particular organism. They also consider more complex issues, such as whether endangered species are worth saving even if doing so severely impacts the local economy, or whether a closed system, such as an aquarium or space station, can really be self-sustaining.

Finally, a tremendous amount of supplementary information about organisms, habitats, niches, and ecosystems is available on CD-ROM, laser disk, and video. Viewing some of these images or using them to create a multimedia presentation gives students exciting opportunities to connect what they are learning in class to the natural world.


If there are not too many herbivores in the tank, students will soon see several varieties of algae growing. While advanced students can key out the specific types of algae, the entire class can have direct experience with the concepts of habitat and adaptation using a much less technical approach. By observing colors, growth patterns, and the location in the tank, all students can easily differentiate one type of algae from another. Where did the algae come from in the first place? Why does a particular alga grow in one place but not another? How does the distribution of algae change over time?

Quite naturally, students will begin to discover how minuscule changes in the environment give one species an advantage over another. Ultimately, rather than conceiving of an ecosystem as comprised of several well defined habitats, students will see that each habitat can itself be divided into many microhabitats.

Identifying the characteristics of the microhabitats is a valuable exercise in problem solving. For instance, one challenge is to figure out how to observe differences in the flow characteristics of the tank's waterfalls, pools, and rapids (Fig. 5). This might be done by watching the movement of strings, which can be attached to glass rods embedded in the gravel or to suction cups mounted on the glass. Long, flexible plants can also serve as natural, qualitative flow indicators. Students could then create their own "wiggle scale" to describe the amount of turbulence.

Another approach might be to release neutral-buoyancy flow indicators ("Cuisenaire" cubes used in math classes, for example) and record how they travel and where they go.

In addition to flow rate, the River Tank's microhabitats are influenced by light intensity, moisture levels, and substrate differences. Once the microhabitats have been tentatively identified, students can collect samples from each site to make direct observations and confirm whether the algae, diatoms, and bacteria are indeed different at each site.

Collecting samples can be done by scraping off small amounts of material, by examining what grows on the flow-indicator strings, or by using settling plates. Set the plates on the terrestrial and aquatic surfaces or attach them to the glass with lettuce clips (suction cups with a plastic clip attached - available at pet stores). Settling plates must be left in place at least one week, preferably longer. Once the samples are collected, students can hone their lab skills by observing and describing the samples under the microscope.

Chemistry in the "River Tank"

Of the waste products fish excrete, ammonia, phosphorous, and undigested food impact an aquarium most significantly. Phosphorous occurs principally as dissolved phosphate and, since phosphate is often a limiting factor in plant growth, it is quickly removed by the plants directly. Undigested food and some by-products of digested food, namely amino acids and proteins, are a major source of dissolved organic compounds - a major component of the detritus that accumulates on the gravel bed and in the filters. This detritus is essentially harmless, but it clogs the filters and coats the gravel, limiting the ability of any carbonate particles in the gravel to dissolve and help buffer the water.

Ammonia (NH3) is toxic to fish and must be removed or converted into benign substances before it builds up to lethal levels (levels above 0.1 ppm are considered dangerous). Some of the ammonia is taken up directly by certain plants (including most algae), but most of it is converted to mildly toxic nitrite (NO2-) by Nitrosomonas bacteria. Nitrobacter bacteria then convert the nitrite into nitrate (NO3-) a relatively benign, useful compound.

Nitrate is quickly removed from the water by plants, which use it as a source of nitrogen to form proteins and nucleic acids. This processing of ammonia to nitrate is part of what is commonly referred to as the nitrogen cycle. Regular testing of the ammonia, nitrite, and nitrate levels gives a good sense of the robustness of the Nitrosomonas and Nitrobacter populations. Testing also provides insight into how well balanced the tank is in terms of the amount of waste generated and the ability of the bacteria to process that waste.

The pH of aquarium water is another important measure because fish are adapted to living within specific pH ranges; bacteria are sensitive to pH levels as well. African cichlids, for example, prosper in alkaline waters with a pH range of 7.4-8.0, while certain tetras thrive in acidic waters with a pH range of 6.0-6.2. If a fish encounters a pH level outside its preferred range, its slime coat can suffer, making it susceptible to disease. Its fecundity drops and, ultimately, the gas exchange in the gill membranes will be so reduced that the fish may suffocate.

Nitrosomonas and Nitrobacter prefer an alkaline environment (pH 7-8), and pH levels much below 6 severely curtail their activity or kill them. Once the bacteria are gone, the toxic ammonia quickly builds up to levels that will kill all the fish.

In an aquarium, acids derive primarily from two sources. The first is when carbon dioxide (directly dissolved into aerated water or released as a respiration by-product) mixes with water to form carbonic acid.

H2O + CO2 * H2CO3 * H+ HCO3- * 2H+ + CO3=

The other is when ammonia undergoes nitrification by Nitrosomonas.

2 NH3 + 3 O2 AE 2 NO2- + 2 H+ + 2 H2O

Should any part of the tank become anaerobic, the heterotrophic bacteria produce through fermentation lactic, acetic, formic, and other organic acids, which will add to the acid load of the tank and lower its pH.

An underappreciated source of periodic pH swings is photosynthesis. During illumination, plants take in carbon dioxide, raising pH as the amount of carbonic acid (one source of carbon dioxide) in the tank is reduced. At night, respiring plants release and thereby increase the amount of carbon dioxide; consequently, the amount of carbonic acid in the tank increases, lowering the pH.

Water hardness is an often overlooked though extremely important component of pH balance in an aquarium. Water hardness refers to the total concentration of calcium and magnesium ions in the water, primarily from calcium carbonate (CaCO3) and magnesium carbonate (MgCO3). These ions, called buffers, are important because they slow the rate at which the pH changes.

The equation below shows that carbonic acid (H2CO3) dissociates into hydrogen (H+) and bicarbonate (HCO3-) ions. The bicarbonate ions can further dissociate into hydrogen (H+) and carbonate (CO3=) ions. When acid (H+) is introduced into well-buffered water, carbonate ions react with the hydrogen ions to produce bicarbonate. Thus, even though acid is added, no change in the overall pH occurs. Furthermore, bicarbonate ions act as an additional reservoir for hydrogen ions. The reactions outlined in the equation below are pH sensitive and shift to the right as pH increases.


If the aquarium water is not well buffered (5.6-11.2 dKH or 100-200 ppm calcium carbonate in a freshwater tank), any acid that is added serves to drive down the pH. Consequently, the daily pH swings caused by photosynthesis can combine with longer-term acid accumulations and cause the tank to suddenly crash because of catastrophically low pH levels.

Oxygen is required to keep fish healthy and active and to maintain an aerobic environment that will support a robust, aerobic bacterial colony. The River Tank's waterfalls and rapids aerate the water and keep it well oxygenated. A comparison of the oxygen levels between the River Tank and a container of standing water would demonstrate the role agitation and mixing play in maintaining oxygen levels. The agitation also helps dissipate any chlorine in the water which escapes at the surface as chlorine gas. Connections to the Science Curriculum The River Tank can be used to enrich a broad number of science topics as well as serve as a source of problem-solving challenges (Fig. 6). For example, when conducting field studies of a stream, scientists draw a cross-sectional profile of the stream and calculate its rate of flow. Students might devise a way to do this using the tank.

Interesting comparisons between the "River Tank" and traditional aquariums can be made. In addition to the comparison of oxygen levels mentioned previously, students can compare the evaporation rates of the two types of tanks or discuss what types of plants (Fig.7) and animals are best suited for each type. Temperature stratification is another important difference between flowing and standing bodies of water. Students can determine if the water temperature in each type of tank is the same throughout, or if it is stratified.

Another curricular tie-in might focus on the water cycle, which is clearly illustrated inside the tank as the vapor from the flowing water condenses and appears as water droplets on the cooler, outside surfaces. Since recirculating the water makes the tank vulnerable to the build-up of minerals caused by evaporation, a class could expand its study of the water cycle by seeing if plants watered with this mineral-laden water grow better than plants watered with tap water. The mineral build-up in the water can be quantified, also.

The "River Tank" provides an effective way to present abstract ideas concretely. For instance, when discussing adaptation, students can immediately see why animals with streamlined body shapes (Fig. 8), and plants with long, flexible stems, live in streams. Concepts such as the water cycle, niches, ecosystems, changes over time, and the importance of the balance between the biotic and abiotic elements are powerfully reinforced by what students see in the tank and measure in their experiments The River Tank serves as a dramatic visual-aid that illustrates complex ideas in a way even young students can comprehend. Further Reading Adey, W., and K. Loveland. 1991. Dynamic Aquaria, Building Living Ecosystems. Academic Press, Inc., San Diego. Andrews, W. A., and S. J. McEwan. 1987. Investigating Aquatic Ecosystems. Prentice-Hall Canada Inc., Scarborough, Ontario. Spotte, Stephen. 1970. Fish and Invertebrate Culture, Water Management in Closed Systems, 2nd edition. John Wiley and Sons, Inc., New York. "River Tank" is a registered trademark of Finn Strong Designs, Inc.

"Field Collection of Living Organisms"

Lisa Darmo, Ph.D.
Genetics Department

While Carolina offers a wide selection of aquatic and terrestrial organisms suitable for stocking your "River Tank" Ecosystem and other habitats, you may wish to collect your own. Field collection is an interesting, educational, and rewarding activity for students, so long as the students follow a few simple but critical guidelines. Students and educators should always be as conscientious and ethical as possible in their collection activities. Failure to comply with county, state, or federal regulations that prohibit the collection of particular plant and animal species, or the collection of any species in a prohibited area, may be punishable by fine or imprisonment. Be sure to check state and county regulations with your local wildlife, forestry, fisheries, and/or natural resources department. Information on federal regulations may be obtained from the U.S. Department of Fisheries and Wildlife. In planning your collection activities, follow these guidelines:

  1. Do not collect any rare, threatened, or endangered species.
  2. Obtain the owner's written permission to collect on private property.
  3. Do not leave signs of your presence at any collection site. Pack out trash and leave no other signs of disturbance.
  4. The Endangered Species Act and some state and local laws prohibit the collection or disturbance of certain plants and animals wherever they are found (whether on public or private property). Many public lands (state and national parks, wilderness areas, recreational parks) prohibit removal of any plant, animal, rock or mineral. Many species may be collected only with the proper permit. Know local and federal regulations.
  5. Do not take all specimens at a site; always leave the majority of the organisms undisturbed. Never take more than you need.
  6. Put rocks, stumps, etc. back in the position in which they were found (moist side down).
  7. Release organisms at the collection site when finished with them.
  8. Only release organisms into an area from which they originated. Even if a species is found in your area, if your particular specimens did not originate in that area, they must not be released to that site.
  9. Treat all animals, collected or purchased, in a humane and ethical manner. Before removing animals or plants from their native habitat, prepare a suitable environment for them and be aware of their needs for survival.

"Biosphere Wonder"

     An excellent resource in you, we find, 
Constantly being produced all the time 
     Passing through an animal's digestive track
Containing many nutrients - this is fact
     Deposited in large quantities and places.
In convenient locations and spaces 
     Made up of undigested remains of plant food 
Plus vast numbers of bacteria chewed 
     And animal waste products as well 
As a broken down blood cell 
     You also attract your own fauna and flora,
Consisting of bacteria, fungi, and protozoa
     Nematodes, annelids and arthropods, too 
So much work in the bioshpere to do
     Rich in water soluble vitamins you are
Growth factors and mineral ions make you a star,
     Disliked by most, yet in the bioshpere a perfect host.
Ding-Dong / Dung
     Your name has been rung
You'll always be around
     We'll look everywhere and you'll always be found.
Dr. Phyllis H. Lanier
September 12, 1994

Book Watch

The Book Doctor

Study nature not books! - Louis Agassiz

From these very first words, Dennis Holley's Animals Alive. An Ecological Guide to Animal Activities (H4-45-3803) provides a refreshing and thorough resource for teachers at all levels. The author's text and Brian Payne's excellent drawings are designed to help teachers develop students' innate curiosity about the natural world and living animals. The goal is to have students collect and study animals, and then release them back into their natural habitat whenever possible. Of course this isn't always practical, and the author provides information about how to purchase animals, also.

Each chapter investigates a major phylum of animals, beginning with the simplest in structure and moving to the more complex. Using the five-kingdom system, the author classifies and describes the animal's structure and habitat. He then explains how to collect and maintain specimens or, if this isn't feasible, discusses the next best options.

For example, after a detailed discussion of mammals, the author writes: "The temptation may be great, but wild mammals should not be collected or kept in the classroom. . . . The definition of wild mammal should also extend to so-called tame mammals such as raccoons, squirrels, and foxes reared by humans. . . Tame wild mammals may have a place at school but for only a brief visit - never as classroom residents and only under the handling and supervision of an expert."

Always maintaining the proper health and safety concerns for teachers, students, and animals, Holley then offers extensive teaching activities organized around the students' observations of the animals and their role in the ecology of the region. Questions are open-ended and stimulating.

The final phase of Holley's program involves returning animals unharmed to the outdoors, whenever possible and if not in violation of the law. He includes names and addresses of commercial suppliers, wildlife and natural resource authorities, state departments of education, state departments of health, and additional resources.

If you want inexpensive live animal activities that are noninvasive and observation oriented, Animals Alive! provides the answers.

The Book Doctor's Choice

Animals Alive!: An Ecological Guide to Animal Activities. Dennis Holley. 1994. 301 pages. This book is a complete guide and resource for designing and conducting live animal activities. Beginning with an excellent introduction, in which the author tells how to collect, maintain, and dispose of specimens, the book provides more detailed suggestions for activities studying invertebrates and vertebrates. Appendices offer a guide to the five kingdoms classification and various sources of additional information and supplies. Many line drawings. Soft cover.
     H4-45-3803     Each . . $29.95

Books and Field Guides from Carolina

Audubon Society Field Guides

Full-color photographs, range maps, and ample descriptions make these books among the best identification guides available. Size is just right to fit the hand. Soft cover.
     H4-45-4499     Insects and Spiders . . $18.00
     H4-45-4806     Reptiles and Amphibians . . $18.00

Peterson Field Guides

Perhaps the most respected nature series of all time, these guides emphasize easy identification by the quick recognition of key field marks. Illustrated with line drawings and photographs. Soft cover.
     H4-45-4714     Freshwater Fishes . . $16.95

Freshwater Invertebrates of the United States

Robert W. Pennak. 1989 (3rd ed.). 803 pages. Comprehensive reference work for aquatic zoologists, limnologists, fish biologists, and entomologists, as well as a thorough text for the college senior and graduate student. Hard cover.
     H4-45-3901     Each . . $79.95

The Practical Entomologist

Rick Imes. 1992. 160 pages. What is an insect? This introductory work defines the term and then describes the general characteristics of the insects' anatomy and life cycle. The author then explains the classification of insects and discusses field techniques - including taking photographs and keeping a notebook. Soft cover.
     H4-45-4507A     Each . . $15.00

Dr. Axelrod's Atlas of Freshwater Aquarium Fishes

Herbert Axelrod. 1993 (7th ed.). 1,151 pages. You need not own an aquarium to appreciate these exquisite color photographs of over 7,500 fishes. Range, habits, water conditions, size, and food requirements are given. Hard cover.
     H4-45-4716     Each . . $89.95

The Completely Illustrated Atlas of Reptiles and Amphibians for the Terrarium

Jerry Walls (editor). 1988. 830 pages. The most complete book available on this subject, this atlas contains 2,000 illustrations (most in color) and information on almost all genera known. Descriptions of behavior and form, as well as an index to common names, make this a valuable reference for amateur and professional. Hard cover.
     H4-45-4989A     Each . . $129.95

"River Tank" Ecosystem

Introducing a dynamic habitat where fish, lizards, frogs, and plants can interact in ecological balance.

So innovative that it won first prize at the New England Inventor's Exhibition at MIT, the River Tank Ecosystem is widely praised by scientists, teachers, and parents. Offering a cross section of a river in a tank, your students will marvel at the shallow and deep pools, waterfalls, caves, plants, and the sound of cascading water. Fish and other animals can move freely from pool to pool and level to level. The same processes that give life to a river take place in your classroom. For example, rushing water generates a healthy, oxygen-rich environment and plants use fish waste as food. The seepage of water through the unique gravel-filled plant cavity stabilizes pH and biological balance.

Because it is an ecologically balanced system, the River Tank Ecosystem requires very little maintenance. Beginners and experts alike can marvel at and enjoy the River Tank Ecosystem. Each ecosystem comes complete with tank, insert, pump, strip light, heater, and teacher's guide that provides simple assembly and maintenance instructions. The 30- and 45-gallon systems include a lizard ledge. Shipped from the manufacturer, please allow 2-4 weeks for delivery. Shipping and handling charges included. Animals and plants are not included.

     H4-16-1561     10-gallon System . . $155.00
     H4-16-1563     20-gallon System . . $285.00
     H4-16-1565     30-gallon System . . $385.00
     H4-16-1567     45-gallon System . . $515.00

Your students will learn all about nature's ecosystems while studying interaction of plants and animals in the "River Tank" Ecosystem. Order your healthy organisms after you set up your tank, or you may specify that they be shipped 2 weeks after you receive your "River Tank" Ecosystem. Reptiles are suitable for 30- and 45-gallon systems.

Dwarf Aquarium Frog

Small aquatic frog ideal for use in aquaria. With instructions.
     H4-L1415     Each . . $2.70;  3 . . $7.49


Aquatic. Triturus (Notophthalmus) viridescens is usually shipped. With instructions.
     H4-L1452     Per 3 . . $7.15;  12 . . $24.15

Ramhorn Snails

The most popular aquarium snail, available year-round. With instructions.
     H4-L489C     Per 3 . . $2.45;  12 . . $8.95


Not sexed. Large and medium sizes are good for laboratory study and dissection; small size is ideal for the aquarium. Please order 10 days in advance. Cannot be shipped into CA.
     H4-L590     Large, per 3 . . $7.80;  12 . . $24.95;  50 . . $91.95
     H4-L592     Medium, per 3 . . $6.90;  12 . . $23.95;  50 . . $64.95
     H4-L593     Small, per 3 . . $4.80;  12 . . $15.95;  50 . . $43.95

Basic Aquarium Set and Plants Set

For the 10- or 20-gallon River Tank. With instructions. *Note: Purchase two Combinations for the 30- or 45-gallon River Tank. Animals Set: 6 Snails, Newt or Mussel, 6 Ornamental Fish, 2 Tadpoles.
Plants Set: Floating Plants, 8 Elodea, 8 Cabomba, 8 Sagittaria.
     H4-16-1392     Animals Set . . $15.35
     H4-16-1394     Plants Set . . $11.50
     H4-16-1400     Combination (one each of above) . . $22.95*

Food for Small Animals

House Cricket

Acheta domesticus. With instructions.
     H4-L715     Adults, per 12 . . $4.95;  50 . . $12.40;  100 . . $19.98
     H4-L715A     Eggs (50-100) . . $5.75
     H4-L715B     Nymphs, per 12 . . $5.05;  50 . . $11.50
     H4-L715C     Life Cycle Set (all stages) . . $8.79


Tenebrio. The adult lays hundreds of eggs and is the best stage to start a balanced culture. Larvae are ideal food for lizards, salamanders, turtles, and frogs. Pupae metamorphose into adults in about 2-4 weeks. With instructions.
     H4-L890     Adults, per 12 . . $6.89; 50 . . $15.95
     H4-L891     Larvae, per 50 . . $5.25; 100 . . $7.75; 500 . . $19.90; 1,000 . . $29.90