

Can You Use Distilled Water in a Fish Tank

can you use distilled water in a fish tank

Taking care of fish is not as easy as it may seem to guests who only see the beautiful fish tanks. There’s a lot of work that goes into keeping the fish alive and healthy. The saline levels of the tank, the filters I have to use, cleaning the tank of tannins or other substances – these are all required. Another important aspect of having a fish tank is the type of water you use. Can you use distilled water in a fish tank? How about purified water, can I use purified water in my fish tank?

The answer is quite complicated. Here’s an in-depth look at the repercussions of using distilled water in a fish tank.

Can I Use Distilled Water in a Fish Tank?

Beginner aquarium hobbyists might think that using the cleanest kind of water must be the safest for the fish. That kind of sounds right, doesn’t it? No, it is not.

What Happens During Water Distillation?

Here’s what happens during water distillation. The water is boiled and evaporated. All the impurities in the pre-evaporated water are left behind as the water turns to steam. Then the steam is cooled down again in a separate chamber and turned back to liquid. The result is water that contains no impurities, just 100% H2O.

Is it Safe to Use Distilled Water in a Fish Tank?

What’s the problem with this, then? Distilled water has no TDS (total dissolved solids). That also means that it contains no minerals whatsoever. If you have been taking care of an aquarium for quite some time already, you know by now that fish need minerals in their water.

If the fish tank contains only distilled water, the water will siphon the minerals out of the fish’s body because of osmotic pressure. Without getting into many details, osmotic pressure will basically kill the aquatic life if the water contains no minerals.

At Times You Can Use Distilled Water in a Fish Tank

when to add water to a fish tank?

The problem with distilled water is it is just practically H2O, and that is not enough for fish and aquatic plants to live. But that does not mean that you completely can’t use distilled water in a fish tank. Here are some scenarios where you can use distilled water.

Topping Off Water Loss Due to Evaporation

As the water in the tank evaporates, the minerals stay in the tank (remember the distillation process?). You can replace the evaporated water with distilled water.

Replacing evaporated water with distilled water does not change the balance of the minerals in your tank. The number of minerals remains the same.

Softening the Water

You can use distilled water to lower the pH levels of your fish tank. This is a tricky process, however. The softening effect of distilled water on hard water (such as tap water) is on a logarithmic scale, not proportional. This means that combining two equal parts of hard water and distilled water will not necessarily halve the pH level of the mixture.

You will have to test the pH level of the water every time you mix parts of the two kinds of water.

Additionally, distilled water also lowers the carbon hardness of the water, It may take you a while to get the perfect mixture of tap water to distilled water for your fish tank. But this is one way you can use distilled water in a fish tank.

Remineralized Distilled Water

There is only one instance where you can use distilled water to replace all the water in an aquarium. That instance is when the distilled water is remineralized.

Actually, this is the best use for distilled water in a fish tank. Remineralizing distilled water allows you to utterly control the mineral content of the fish tank. Because distilled water contains 0 TDS, the only minerals in the water will be the minerals that you put there.

Can I Use Purified Water in My Fish Tank

Purified water is any water that has undergone a purification process, such as distillation or de-ionization. For water to be considered purified, it should contain no more than 10 ppm (parts per million) of TDS.

I don’t suggest using purified water unless you know what dissolved solids it contains. If you don’t know whether it has impurities or the right minerals, I suggest testing it first before using it for your fish tank. And if you use purified water, I advise you to adapt it to the needs of your fish.

Can I Use Spring Water in My Fish Tank

The only requirement of spring water is that it should have come from the ground. With that said, there is a lot of unpredictability to random spring waters. You don’t know what’s in the water. You don’t know what minerals or impurities are in there unless you get it tested.

I don’t suggest using spring water for your fish tank without due diligence on your part. You need to know what the contents of the water are. You might have to do a bit of trial and error, but that is kind of risky.


Can you use distilled water in a fish tank? Yes, but only on certain occasions. If you need to replace water lost due to evaporation, distilled water is the best option. You can also use distilled water to soften hard water. Mineralized distilled water is also the best type of water to use in a fish tank, in my opinion. As for spring or purified water, I suggest adapting them for the needs of your fish. It may take a while, though, considering that not all spring water and purified water are the same.

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