Your Aquarium Is Overstocked – Now What?

What can you do if your aquarium is overstocked?

If you’re a fishkeeper and you’ve been online for five or ten minutes then you’ve seen this in the comment section once or twice:


overstocked goldfish at a store

And if that’s gotten you worried that maybe your fish tank is overstocked, then this article is for you—I’ve got some advice you might not often hear elsewhere on the internet.

Just Get A Bigger Fish Tank?

People will insist, almost to the point of bullying, that you must get a bigger tank otherwise you’re a terrible human being. Certainly, I cannot deny it, because a bigger tank is always better for your fish. Of course it is. When you’re dealing with a larger body of water, it’s more difficult for you to mess it up; i.e. ammonia, nitrite, and nitrate take longer to build up to dangerous concentrations in the water.

There are of course other benefits to larger tanks: more swimming space, reduced aggression between quarrelsome fish, etc.

Anyway, now that I’ve established with you that if you can get a bigger tank, you probably should.

overstocked aquarium what to do
Aggression can be a problem in overstocked tanks–check out our article on how to manage cichlid aggression!

But I say “if” for a reason.

Because not everybody out there can get a bigger tank, and I don’t think we—in the fishkeeping community—should ostracize people who plainly aren’t in a position to spend the money on a monster tank. Or maybe they don’t have the space? They could be in a cramped city apartment, or a kid whose own space is restricted to the four walls of their bedroom.

My Aquarium Is Soooo Overstocked… Should I Just Give My Fish Away?

I understand what comes next, some of us would say they should rehome those fish? Nah, I don’t believe in telling people to rehome the fish they love. You could argue it’s best for the fish, and you might be right.

But I say “might” for a reason.

The reason is that there is an alternative to getting a bigger tank if your tank is considered “overstocked”.

Who Doesn’t Like Clean Water?

Just change the water more often. And change larger amounts.

overstocked aquarium clean gravel
Don’t forget to clean your gravel when you do a water change,

It sounds simple, and it can be simple, but it really depends on your resolve. If you want to overstock your aquarium, then you simply must dedicate yourself to what some might see as an extreme water change regime.

Fish love clean water; that’s water with as close to zero nitrogen compounds as possible (ammonia, nitrite, nitrate). The best way to provide them with that is to empty out a portion of the old water, and replace it with clean water from your tap. Don’t forget to add your dechlorinator! A water change, I’m talking about the humble water change.

How Frequently Should I Change the Water In An Overstocked Aquarium?

How often you’ll need to change your water if your tank is overstocked will depend on a lot of factors.

  • Just how overstocked is the tank?
  • How messy are the fish you have? (Think: oscars, plecos, goldfish).
  • Is your filter large enough to get ammonia processed quickly enough?

So, I’m not really going to be able to tell you do “X water changes, of Y percent, Z times a week” because that’s just not possible. People who tell you it is are oversimplifying it.

Testing your ammonia, nitrite, and nitrate is an important part of maintaining an overstocked aquarium
API’s excellent Freshwater Master Test Kit — An important tool if your tank is overstocked. You can pick up the test kit on Amazon here (affiliate link).

Therefore, the only hundred percent way to know for sure how often to change your water is to test it for ammonia, nitrite, and nitrate on a regular basis and see how quickly they build up in your tank? If your filter has enough surface area for beneficial bacteria, then you should see minimal ammonia/nitrite, but the nitrate will build up.

But, again, money intervenes. We might not all have the cash flow to direct to water testing kits. So, in that case, I would just say to monitor your fish. They will let you know if you’re not changing your water enough.

overstocked aquarium sick fish
By Alex Popovkin, Bahia, Brazil from Brazil – Astronotus crassipinnis (Heckel, 1840),
CC BY 2.0,
A poorly maintained and overstocked aquarium will almost certainly lead to sickly fish.

Ideally you don’t want it to get to the point where your fish are looking sickly because you haven’t given them enough clean water, so just change it as often as you can.

To put it into perspective, some of the best discus breeders all over the world do between 50% and 100% water changes… DAILY. Yes, every day.

Think Long and Hard Before Overstocking Your Aquarium

So, in conclusion, I’d like to really emphasize that I do not recommend people overstock their aquariums. A lot of us might say to ourselves that we’ll compensate with lots of water changes, but in reality, we might not keep that promise to ourselves and our fish. And it’s the fish that will suffer.

I say, think long and hard before overstocking your tank.

But, if you’re committed to taking care of your fish, an overstocked aquarium can be done!

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Good Quality Food Helps Maintain Water Quality Too!

One thing to consider is the quality of the food you feed to your fish. Because low quality food can really make a mess in your aquarium. I’ve put together a list of some of my favorite foods to feed my fish and why I feed them that. Check it out here.