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How to Add Water to a Fish Tank

how to add water to a fish tank

How to add water to a fish tank without damaging anything in it, that is an excellent question. No one can expect to throw a bucket of water into the tank willy nilly and expect the fish to be all good with it.

In this post, I will share the best method on how to add water to a fish tank. I will discuss the thought process, the best kind of water to use, and the reasons why you should add more water to a fish tank.


How to Add Water to a Fish Tank the Safe Way

There’s a lot of consideration that goes into adding water into your aquarium. Here are the questions I ask myself before I pour water into my fish tank.

Choose the Right Water

There are four main aspects of water that I check for when adding water into my fish tank. These aspects are alkalinity, pH, nitrates, and phosphates.

The alkalinity of water determines its capability to neutralize acids and keep the water at a stable pH level. And just so you know, a lot of fishes prefer to have their water non-acidic.

The pH level of water measures how basic or acidic it is. Without getting too much into the scientific and chemical details, let’s say that particular aquatic life thrives in specific ranges of pH levels. For example, most tropical aquarium fish survive only in water with a pH of 6.8 to 7.6.

Nitrates in a fish tank have repercussions. Too many nitrates is an open-door invitation for an algal bloom. Or worse, it can lead to nitrate toxicity for the fish, eventually killing them.

While phosphates are not harmful to fish directly, too much of them can lead to a bad environment for the fish. Phosphates are the product of natural decay and waste. Uneaten food and fish waste contribute to the number of phosphates in the aquarium. Like nitrates, phosphates encourage algal bloom and will eventually reduce oxygen levels in the tank. Too many phosphates will suffocate the fish.

What do all of these aspects have to do with adding water into the tank? If you add water with very high acidity or filled with too many nitrates or phosphates, you are basically goading your fish to die.

Personally, I prefer using distilled water when I’m adding water into the fish tank. Distilled water has no dissolved solids – it is just pure H2O. Because distilled water has no solids, it will not affect the mineral content of the tank. If I add it to the fish tank, the number of nitrates, phosphates, and minerals in the tank remains the same.

I still have to measure the pH and alkalinity levels of the tank after adding the water, though.

If you are using tap water, I suggest you water conditioner first. The water conditioner removes the chlorine from the tap water.

Use a Clean Container

Use a clean container to add water into the fish tank. The container should never have soap, NEVER. To be sure, rinse the container thoroughly.

Slowly Pour Water into the Tank

Even while the fish is in the tank, you can safely add more water to it. “How is that possible?” You may ask. As long as you don’t highly agitate your fish, they will not get too stressed about you adding more water. Just as fish don’t mind an aquarium pump that creates movement, they will not mind the slow flow of water going into their tank.

Besides, dropping water slowly into the tank oxygenates the water. Again, I stress, do this VERY slowly to avoid agitating the fish.

Test the Water

After adding new water into the tank, test the water for alkalinity, pH, nitrates, phosphates, and other mineral levels. This last step is just to make sure that the new water settings in the tank will not harm the fish.

When to Add Water to a Fish Tank?

when to add water to a fish tank?

What are the reasons for adding water to the fish tank? And when is the best time to do it?

NOT to Make Space for New Fish

As a general rule, you should not have too many fish in a tank. So you should never add more water to the tank so that you can add more fish.

Water Evaporation

The primary reason why I add water to my aquariums is that the water evaporates. Water evaporation is also why I prefer adding distilled water – all the mineral content of the tank remains constant even if the water evaporates. When I add distilled water to replace the evaporated water, the tank’s phosphate, nitrate, and mineral content stay the same.


The steps I enumerated are the best process on how to add water to a fish tank. I put a great deal of thought into the kind of water I put in the tank more than how I will put it in the tank. Once I got the suitable water, I simply slowly let the water drip into the tank. I don’t do it quickly, so I don’t agitate the fish. Then, I test the water to make sure that my fish have the right environment.

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