Why is my cory catfish swimming at the top of the tank?

Last Updated on July 14, 2021 by cmoarz

If you’re watching your corys and notice some weird behavior where they will jolt up to the top of the tank and back down again, Or even sit at the top of the water, there is a good reason for this behavior.

There are actually 2 reasons they do this. The first is common healthy behavior, where they see something they potentially want to eat and just fly up real quick to the surface to grab it, and then come back down.

This is normal behavior and nothing to worry about, and even though they are bottom feeders, they do occasionally like an opportunistic snack.

The other reason is more cause for concern and has to do with the available dissolved oxygen levels in your aquarium water.

There are a lot of misconceptions when it comes to dissolved oxygen levels in a tank, some of them are:

1. “If I keep my tank cool, then the dissolved oxygen levels will be adequate.”

2. “I don’t need to worry about learning about dissolved oxygen in aquariums because it’s too complicated for me.”

3. “When I do water changes, if the tanks already full of fish and plants, then this amount of water exchange won’t affect the DO levels at all.”

Most of these are simply untrue or only partially true. The fact is, You almost certainly do not have enough oxygen to keep your fish happy if you aren’t using air stones or other oxygenation techniques.

Nearly all fish will stop eating and show signs of distress if the dissolved oxygen levels are too low. Fish that have access to surface air may not show these signs because they can simply come up for a breath and get enough oxygen.

As bottom feeders, corys are a really good way to spot an issue before it becomes a bigger problem.

Fish need a constant supply of oxygenated water to live successfully as they can only take so much from what’s available at the surface. If there’s not enough in the water, your corys and other fish will sit at the surface and gasp for air.

The simple fix is to add an airstone or 2. Depending on the size of your tank, you may need more than one for adequate oxygenation.

If you rely on a filter to keep the water clean and oxygenated, then they are simply not enough. You also need something that is going to allow dissolved oxygen in the water to replenish itself quickly, and this can only be achieved by physically agitating the water and injecting air into it.


Owner of AquariumGravel.com and also owner of actual Aquarium Gravel believe it or not! ;). Setting up beautiful aquarium sceneries and habitats since I was very young. Enjoy!