How Often Should You Change Your Aquarium Water?

Water changes are an essential part of the maintenance of our aquariums. Because an aquarium is a closed system, we as fish keepers have to take care that everything in the tank is perfect for them. But doing water changes take time, and without a doubt every fish keeper has asked themselves the following question at some point in time.

How often should we do water changes in our aquarium? For a averagely stocked tank, you should do weekly 15 – 20% water changes. This will make sure the water quality in your fish tank stays pristine. For heavily stocked tanks a weekly 20 – 30% water change is needed and for lightly stocked tanks a bi-weekly 20% water change will suffice.

But what factors should you take into account when determining how often you should do these water changes? Things like what kind of fish, the amount of food you feed and how many live plants are in the tank all play an important roll in the water chemistry.

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Why Do Water Changes

Water changes allow us to refresh water that contains waste and toxins with fresh clean water. This has to do with the nitrogen cycle, where fish, leftover food and decaying plant matter are a source of ammonia. This ammonia is broken down into nitrite, which again is broken down into nitrate.

If the nitrogen cycle is functioning well in your tank, you will have 0 part per million (ppm) ammonia, 0 ppm nitrite and < 100 ppm nitrate. This is a desirable situation because nitrate is not nearly as toxic in low concentrations as ammonia en nitrite.

Nitrate will slowly build up over time, and although it is not as toxic as the other two substances, we do not want it to reach levels at which it will become lethal to our fish. Therefore we do water changes.

Can You Do Fewer Water Changes When You Have Live Plants

Next to doing water changes to take the nitrate out of the tank, plants help us with removing nitrate and ammonia from the water as well. This is perfect! They essentially help us clean the aquarium. I wrote an article that goes more in debt, you can read that here.

So nitrate is removed from the water column by plants, does that mean that we can do fewer water changes? That definitely is possible, but they are by no means a substitution for water changes.

Let’s assume that they take out 90% of the nitrate produced in a week. That is a lot, but there will still be 10% of the nitrate left that the plants did not manage to consume. This will build up, and after a couple of weeks your nitrate level will have been climbing to a level where you still have to be aware of toxic concentrations.

So when you have a (heavily) planted fish tank, you can do fewer water changes. Maybe even once every 2 weeks or once every 3 weeks. It does however not mean that you never have to do water changes again, because nitrate can accumulate in your tank.

Stocking and Feeding

A big factor in the period between your regular water changes is determined by what fish you are keeping in your aquarium. If you keep bigger fish, maybe even some fish that are considered ‘monster fish’ like red tail catfish, giant gourami or peacock bass, but also already fish like convict cyclids or oscars, you have to do more water changes.

These fish are super messy eaters, and you have to feed them a lot! Also, you can not get away with feeding them dried food, they need frozen food or pieces of meat. This kind of food is devastating for your water quality, and you have to keep up with water changes.

If you keep just a couple of small fish that have relatively a lot of space, your water condition will remain good for a longer amount of time. I for example have a 20 gallon (80 L) fish tank in which I keep a couple of tetras and a bristle-nose catfish. Because I keep a small amount of fish I do biweekly water changes of 20%.

You can influence your water quality and how long it stays good by being careful with what you feed, and especially with how much you feed. If you only feed what your fish can eat, your water quality will remain better than when you feed more than your fish can eat. All uneaten left-over food will decay and become an ammonia source.

Is Topping Off Sufficient

The question “can’t you just top off your aquarium?” comes up quite a lot, usually by fish keepers who are tired of doing water changes and basically are too lazy to take care of their fish. They top off their tank because a portion of the water that was in it has evaporated.

When you are only topping off your aquarium, and not actually taking out any water beforehand, there is no way that nitrate will leave your tank. If there is not a way for the nitrate to exit you are only relying on your live plants, and that for most of us is not enough.

Topping off is the number one cause of old tank syndrome, where you accumulate your nitrate over a longer period of time up until the point where the concentration becomes toxic for your fish.

At first, your fish will seem to be doing fine, but as soon as you add a fish from the store it will die. Most of the times this is blamed on the fish from the store, but it is actually your tank that is not livable for your fish.

Gravel Vacuum and Clean Filter

While doing your water changes, you should vacuum your gravel. This way, organic plant matter, left-over food and fish feces will be removed from your tank. If not, they will slowly decay and again become an ammonia source.

While you are doing this however, you are also disturbing the beneficial bacteria that are living on your substrate. Up until the point where they are no longer capable of breaking down ammonia into nitrate. That should be no problem because there are still more than enough bacteria housed in your filter.

However, make sure you do not clean your filter AND gravel vacuum at the same time. If you do this, you risk the chance of disturbing both the beneficial bacteria on the substrate and in the filter. This results in completely loosing the tank’s capacity to handle ammonia.

Results are an ammonia spike which is lethal to your fish. This is a little bit off topic but I wanted to remind you all that this is a possibility that can and will happen if you do this.