Can A Fish Tank Be Too Full?

Too little water in your fish tank is a much bigger problem than overfilling it. It can cause unstable water chemistry and lead to a rapid rise in toxic substances like ammonia. But that’s not to mean you should be filling your fish tank to the brim; that can cause several problems as well, though not as bad as those caused by underfilling.

In this post, we explain how high you should fill a water tank and what issues you could run to if it’s too full.

How High Should You Fill a Fish Tank?

Fish tanks are sized by volume, which refers to how much water the tank holds when it’s filled to the brim. So a 15l fish tank holds that much water when it’s completely filled. But that doesn’t mean that you should fill it to the very top.

The optimal level to fill a fish tank is 2.5-5cm (1-2 inches) below the rim. If your fish tank has a plastic trim around the top, use that as a level guide. Add water until it is about an inch below the trim. If you have a rimless tank without the trim, fill water to about 2 inches below the top of the tank.

If the water sometimes leaks over the sides of the tank or splashes out, then you might have overfilled your tank. But water leaks are not the only reason to avoid overfilling a fish tank.

Why You Should Not Overfill a Fish Tank

1. Prevent Fish From Jumping Out

Many types of fish are natural jumpers. They usually jump to escape predators, escape an unhealthy environment, avoid bullying, or just as part of their swimming. Of course, they have no idea that jumping out of the tank leads to certain death if there’s no one nearby to immediately put them back.

Keeping the water level a couple of inches below the top of the tank reduces the risk of fish jumping out. But if you have extra-jumpy fish like betta or rainbowfish, that may still not be enough. The best solution is getting a fish tank cover or net.

As a bonus, an aquarium cover reduces evaporation, keeps dust and debris from the fish tank, and keeps out curious or hungry pets.

If you want to keep your fish tank open at the top, the other option is to lower the water level by 5-6 inches. That way, the fish don’t get out even if they jump. But you have to make sure that the volume of water inside is sufficient for the type and number of fish you have. If necessary, get a larger or taller tank so that you still have enough water even when you lower the water level.

Tip: If some of your fish have a tendency of jumping, it’s a good idea to investigate why. Test the water more often and look for signs of stress in the fish caused by overcrowding, poor water quality, or some other issue. Here’s a great video that goes into more detail about why fish jump out of the water. 

2. Gas Exchange

Gas exchange, where CO2 diffuses into the air and O2 dissolves into the water, is important in maintaining sufficient oxygen concentration in the fish tank. Gas exchange happens primarily on the surface of the water.

Leaving an air gap above the water improves gas exchange, particularly for covered fish tanks. It provides enough space for gas exchange to happen, which ensures the water has adequate dissolved oxygen.

3. Splashing Buffer Zone

Water in a fish tank is always moving, whether it’s because of the filter, an air stone, or active fish. If the tank is filled up too high, this movement can sometimes result in water splashing or spilling out of the fish tank.

This can cause a mess around the fish tank and the water may damage equipment that’s hanging or placed outside the aquarium.

Reducing water level by an inch or two provides a splash buffer zone without having to cover the fish tank with a lid.

4. Space for Equipment

Some fish tanks have a cover or canopy with lights, automatic feeders, and other equipment attached to it that can get damaged by water. If you have such a setup, you’ll need to make sure water from the tank cannot reach the cover when it splashes. Keeping the water level below the rim ensures any electricals above the fish tank are safe.

5. Easier Maintenance

Dealing with a fish tank that’s completely topped with water is a headache. You’ll likely create a mess every time you vacuum the substrate, change the water, and do other maintenance tasks.

Unless you want to do a cleanup every time you service the fish tank, do not overfill it.

Final Thoughts

Overfilling your fish tank is typically not dangerous to your fish. But it can still cause some minor issues that are best avoided.

The only situation where you may want to fill a fish tank close to the top is if it evaporates quickly. This is a common issue with open top tanks, wide tanks (the larger surface area causes more evaporation), and tanks in spaces with low humidity.

To avoid having to constantly refill the water, fill it almost to the brim. This leaves enough water in the tank, even after some evaporation.