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Help Your Marine Fish Thrive

Tips on marine fish nutrition, stress, quarantine and more.

By Scott W. Michael |

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When I was in the retail aquarium business, it was always depressing to see an enthusiastic hobbyist set up his or her first marine aquarium, only to tear it down a year later (or sooner). While many factors contributed to their loss of interest in the marine aquarium hobby, most neophyte aquarists gave up because of disease and parasite problems with their saltwater fish. Fish death is depressing enough to both new and old aquarists, but an additional frustration is seeing money (in the form of a dead fish) go down the toilet. These fish deaths can also seriously dent one's conscience.

Although there is no way to prevent all premature fish deaths, there are ways to decrease the likelihood that this will happen. In this article, I will present some tips on reducing the fish mortality rate.

Reducing Stress
When it comes to keeping saltwater fish healthy, there is nothing more important than minimizing stress! Almost all the problems we see in captive fish can be traced back to this issue. Stress can cause short-and long-term physiological changes and can result in hormonal changes, altered respiration, osmoregulatory and metabolic disturbances, elevated blood sugar levels and more. Some saltwater fish rebound from stressful encounters; others may die days or even weeks after the initial stress was induced. Stress factors also cause immune suppression, which can lead to parasitic and bacterial infections. The saltwater fish you purchase from your local store have already been subjected to a lot of stress. It's important to do all you can to provide the fish with a stress-free environment once they arrive in your aquarium.

quarantine tank
Sixline grouper, clownfish and puffers in a quarantine facility for saltwater fish. Photo by Oliver Lucanus
There's no doubt that different reef fish can withstand varying levels of stress. This may be one of the more important determining factors when considering how well a species does in your aquarium. For example, some do not do well for most aquarists in Nebraska (where I live), yet will thrive when kept by hobbyists who live where the saltwater fish are collected. For example, Hawaiian aquarists tell me they have good success with Hawaiian longfinned anthias (Pseudanthias hawaiiensis), but P. hawaiiensis shipped to me usually do poorly. In many cases, they never seem to recover from the stress induced during the shipping process.


One of the best ways to ensure success with the saltwater fish you purchase is to select species that are more likely to bounce back from stress. You can get information on how hardy a fish is from books available at your local fish store, the library and on the Internet. Go to one of the many marine aquarium chat rooms, and ask fellow hobbyists what experience they have with a particular saltwater fish. Compare their answers with what you find in books and the information you get from your local fish store owner. With all this information, you should be able to make educated choices.

The best way to prevent your display aquarium from becoming a saltwater fish death trap is by quarantining your fish. I can already hear many of you groaning that this is going to cost more money and take up more space. Yes, it will, but it will also be some of the best money you've ever spent. Properly quarantining your fish is essential to long-term success in maintaining a healthy saltwater fish community. After more than 30 years in the hobby, I am convinced of this.

If you are keeping a reef aquarium, proper quarantining is even more important because if a fish gets sick, it's difficult to treat it in an aquarium with invertebrates. Also, a sick saltwater fish can be difficult to remove from a large reef aquarium. By quarantining your fish, you will have the opportunity to observe —and medicate— them before they are introduced to the display aquarium.

Quarantining must be done right to be effective. First let's consider the aquarium itself. I prefer using a 10- or 20-gallon aquarium that has no substrate (you may need a larger quarantine aquarium for larger, more active fish). The simpler the better — you will want to tear the aquarium down and sterilize it after each quarantine period.

The aquarium needs a light (on a normal day-night cycle), so you can observe your saltwater fish, along with a heater and thermometer to maintain proper water temperature. For biological filtration, you can use a sponge filter, or a power or canister filter filled with plastic filter media (do not place crushed coral or carbon in these filters). A sponge filter located in the sump of your display aquarium or a functioning power/canister filter can be moved to the quarantine aquarium when you're ready to get new fish.

After the quarantine period is completed, sterilize the filter and filter media before placing it back on the display aquarium. To do this, soak the sponge filter or media, and the inside of the filter in a mild chlorine solution (2-percent chlorine, the rest tap water) for 24 to 48 hours, then rinse it repeatedly with freshwater until the chlorine smell is gone. To ensure all the chlorine has been eliminated, throw the sponge or filter media in a bucket of freshwater, or place some water in a power or canister filter, and add chlorine remover.

If you do not have an established biological filter, you can just add an air source (e.g., air stone) and do frequent water exchanges (every second or third day, change about 10 to 15 percent of the water) to prevent the build-up of waste products. Although you need to make sure the fish are properly fed during the quarantine period, it is important to not overfeed. Remove any uneaten food from the aquarium immediately.

The quarantine aquarium should have plenty of hiding places so that the saltwater fish feel secure and are less stressed. Plastic flowerpots with a hole cut in one side and a rock set on top to weigh them down work well, as do sections of PVC pipe. Stay away from hard coral skeletons and any calcium carbonate substrates (e.g., crushed coral) because they pull some medications out of solution and are more difficult to disinfect.

Your quarantine aquarium should not be set up near the display aquarium, and it should have its own utensils (e.g., net, thermometer, scrubber pad). The reason for having separate equipment is to prevent the transfer of parasites from quarantine to display aquarium. It's also possible to transmit pathogens from one aquarium to the other with your hands. To prevent this, vigorously wash your hands after working in the quarantine aquarium.

Between each quarantine period, all utensils, decor and the aquarium should be disinfected. Fill the aquarium with water, place the utensils and decor in it, add a small amount of chlorine and let it set for 24 hours. Then remove the water and rinse the aquarium and accessories thoroughly, and soak them in tap water with added chlorine remover.

Keep your saltwater fish in quarantine for at least three weeks. Observe them carefully during this period and promptly treat them if a problem arises. If treatment is necessary, leave them in quarantine at least 10 days after it is completed to ensure they have fully recovered. If a new fish is added to the aquarium while another is still being quarantined, you should start the whole three-week process over again.

As a prophylactic treatment, some hobbyists will drop the specific gravity in the quarantine aquarium to about 1.011 for approximately two weeks before moving the fish to the display aquarium. Many parasites will die at these low salinity levels. After the two weeks gradually increase the specific gravity of the quarantine aquarium before moving the fish. You can do this by replacing some of the water in the quarantine aquarium with sea water that is somewhat more saline than normal over a couple of days. Keep a close eye on the specific gravity of the quarantine aquarium, and slowly raise it to match the specific gravity of the display aquarium.

Even when you are not planning on buying new fish you should always have all the components of your quarantine aquarium at the ready. If a saltwater fish gets sick in your display aquarium, the quarantine aquarium can serve as a hospital aquarium. It's always best to remove a sick fish from the display aquarium to ensure its tankmates do not come down with the same parasite problem.

UV Sterilizers
Although relatively few aquarists seem to invest money in them, an ultraviolet (UV) sterilizer can be a very useful tool to help control pathogens in the home aquarium. Sterilizers use a UV lamp that produces wavelengths of light around 250 nanometers. This light produces energy that kills bacteria, viruses, fungi, microalgae and small protozoa. It does so by disrupting the genetic material of the organisms exposed to the light. Although sterilizers do not kill parasites that are on the saltwater fish, they can be effective in destroying microorganisms that could infect the fish. A UV sterilizer will help to reduce the numbers of bacteria and parasites in an aquarium but it will not completely rid an aquarium of these potential pests.

There are several different types of UV sterilizers, but the most common, easiest to use and safest are the jacketed models. These consist of a quartz sleeve sealed in a PVC tube. Within the sleeve there is a UV lamp. The water passes into one end of the PVC tube and moves along the quartz sleeve as it flows out the other end. The sterilizer can be set up so it is inline with the aquarium filter or sump, or it can be located so water is pumped from the aquarium or sump through the sterilizer unit and then returns directly back into the aquarium or sump. UV irradiation is often used in multitank systems to prevent the transfer of parasites from one aquarium to another (you will most likely see them in aquarium stores that have aquariums on a central filtration system). Water exiting the filter is irradiated before it returns to the aquariums.

There are several factors that will affect the performance of a UV sterilizer. Here are some things to consider when selecting, setting up and maintaining your sterilizer to ensure it works most effectively.

Purchase a large enough unit. The type of sterilizer I use has manufacturer recommendations of 8 watts on aquariums up to 50 gallons, 15 watts for aquariums up to 75 gallons, 25 watts on aquariums as large as 100 gallons, 40 watt for up to 200 gallons and 57 watts up to 220 gallons (other sterilizer manufacturers may recommend different wattage-to-gallon ratios). I prefer buying a more powerful unit if my aquarium is near the recommended limit for a sterilizer. For example, if I had a 75-gallon aquarium, I would get the 25-watt unit rather than the 15-watt version. This will ensure the UV lamp is effective for the amount of water to be treated.

The water must pass slowly enough through the unit to ensure larger parasites are killed. The flow rate, as well as the total volume of the system, will greatly affect the efficiency of the sterilizer. The exposure time of the organisms must be about 1 second or longer (depending on their sizes). To ensure that as many are killed as possible, you should use a maximum flow rate of about 25 to 40 gallons of aquarium water per hour per watt. Of course, the more water that flows through the sterilizer, the more parasites that will be subjected to the light, but larger microorganisms may not be killed.

Replace the lamp frequently. Unfortunately, the efficiency of a UV lamp declines over time. Most experts suggest changing out lamps every six months to ensure maximum efficacy. If you do not keep your sterilizer on at all times (they will heat the water), you will need to keep a record of how often it is being used so you know when to replace the lamp.

Clean the quartz sleeve. Regular glass prevents the penetration of the UV wavelengths required to kill bacteria and parasites. So UV units are equipped with quartz glass sleeves that allow UV radiation to pass through but prevent aquarium water from directly contacting the lamp. Keep this sleeve clean of slime — otherwise the effectiveness of the lamp will gradually diminish. This will require taking the unit apart and cleaning this sleeve every other week or so. There are units on the market that are equipped with a built-in cleaning wiper, which allows you to clean the sleeve without having to take the unit apart or even turn it off. You want a sterilizer that is easy to work with (my favorite is the Aqua Ultraviolet).

There are safety considerations to keep in mind. It is best not to add medications to the aquarium when the unit is on because UV light may alter the structure of some chemicals. Also, never look at the lamp, which can severely damage your eyes. Unplug the unit before working on it and when doing water changes (the light is likely to break if it is hot and comes in contact with cooler water).

Although UV sterilizers may kill some smaller microorganisms that are beneficial to a reef aquarium, you can certainly keep healthy corals while using one. If you like fish in your reef aquarium, I think it is worth the trade-off to ensure they remain healthy.

Check Out Chillers
Many aquarists would increase their success rate with saltwater fish if they had a chiller on their aquarium. A chiller is a refrigeration unit that will prevent aquarium water from rising above a certain temperature. Most of us have heaters in our aquariums to keep the water from getting too cold, but few of us have any way to prevent the water from getting too warm. High water temperature is a problem that is especially inherent to reef aquariums. This results from the powerful lights placed over the aquarium and any pumps located in the aquarium or sump.

The temperature in many reef aquariums (especially in summer) will hover in the mid 80s Fahrenheit. Although there are water temperatures this high in certain reef habitats, I do not believe they are optimal for a home aquarium. I recommend maintaining water temperature between 78 and 82 degrees if you have a reef aquarium, and 76 to 80 degrees for a fish-only setup. By maintaining your fish at a lower water temperature you will slow down their rate of growth (this can be useful if you're keeping larger saltwater fish) and, more importantly, retard the reproductive cycle of any parasites that may be in the aquarium. This will give you more time to react to any signs of parasite infections before they get out of hand.

Another advantage to keeping the water temperature lower is the water will have more oxygen-carrying capacity. It's also important to avoid large or rapid fluctuations in temperature. This is where a heater and chiller will prove their worth.

Don't Neglect Nutrition
In the past, the nutritional requirements of captive marine fish were often overlooked by hobbyists. I first entered the hobby in the 1970s when most of us fed frozen brine shrimp and the occasional flake food. Few of the marine hobbyists I knew considered that we might not be providing our fish with all the nutrients they needed to live long, healthy lives. In retrospect, it is no wonder that so many of our fish faded in color and developed unusual maladies.

Fortunately, much attention is now focused on diet. The modern-day marine fishkeeper has access to many great fish foods, including nutrient-rich flake foods, frozen preparations, frozen mysid shrimp, and fresh and frozen seafood. Some of the fish foods available have added pigment enhancers that will aid in maintaining a saltwater fish's vibrant colors. One of my favorite foods for smaller fish or fish that feed in the water column (e.g., anthias, gobies, dartfishes, firefishes) is Cyclop-eeze. It is a very nutritious, color-enhancing food that stays suspended in the water column for a long time.

I believe that if you follow these tips, you are less likely to become frustrated with this wonderful hobby as a result of premature fish deaths. There are, of course, other things that will be required of you as a fishkeeper to ensure that your fish thrive, but by reducing stress, quarantining your fish, using an ultraviolet sterilizer and a chiller, and by providing the fish with the nutrients they need, you will greatly increase your chances of success. Until next time, happy fish-watching!

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Help Your Marine Fish Thrive

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Reader Comments

Ricardo    Ridgewood, NJ

5/19/2010 9:15:33 PM

Do uv sterilizers harm the beneficial bacteria involved in the nitrogen cycle? If not, how?

Goober    Mayberry, NC

9/12/2009 6:30:51 PM

Good Informaiton

Chuck    Hon, HI

9/10/2009 11:23:34 AM

Very Important information

Sheryl    Bloomington, IN

9/21/2008 7:48:41 AM

Very informed and detailed article

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