What does it mean when fish swim to the top of their tank and start opening and closing their mouth super fast?

Answer 1

It means that the water isn't aerated well enough and the fish is trying to get more oxygen by sucking air down.

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This can also mean that the goldfish is begging for food. This is a learned bahaviour, and is more likely if you hand-feed your fish. Be very careful not to over-feed your goldfish.

Answer 2

This usually means there is not enough oxygen in the tank for the fish. The water needs to be constantly moving, like a river. Waterfall Filters and bubble makers help in this by producing a flow of water. Without them the water becomes stagnant like a pond and makes it hard for the fish to breath. Add in the fact that your fish produce waste and it can build up and kill your fish. If you don't have a filter or bubble maker I suggest you get one as soon as possible. If you do have one and they are still at the top of the tank I suggest doing a water change.

Answer 3

It is making a bubble nest which means it thinks it's habitat is nice and good enoug to have baby betta