Do some of your fishes seem to be staying at the water all of the time? Whenever they try to swim downwards, do they seem to float back to the water surface? Or sometime they’ll even swim upside down as if they’re dead. If so, they might be suffering from a swim bladder disease/disorder! Of course, this does not apply to natural water surface dwellers.
What is a swim bladder?
Also known as the gas bladder or air bladder, it is an internal organ of a fish that controls its buoyancy. It is a gas-filled sac with walls that are impermeable to gases. By controlling the amount of gas in this sac, the fish is able to control its buoyancy.

Cause
There can be different causes, and different causes require different treatment.

Bacteria – A bacteria attack could cause inflammation at the epithelium of the sac, making the sac walls too thick for proper gas diffusion. Thus, the fish is now stuck at certain buoyancy, making swimming very difficult.

Diet – Feeding low-quality food that soaks up water and expand inside the fish can cause food impactions.

Shape of fish – Globoid-shaped fishes such as the Pearlscale goldfish are especially prone to the swim bladder disorder due to their guts being all squashed up in their abdomen.
Treatment
First, tend to your water’s quality. Making sure the water quality is top-notch allows us to assume that it is not a bacteria attack. But if you strongly suspect it to be the cause, then do visit your local pet shop for an appropriate medication. For treatment towards the other forms of cause, first stop feeding the sick fishes for a few days. Fishes can go without food for up to 10 days, so stopping a few days is really all right. After the few days, if the fishes do not get better, try feeding peas. Yes, peas that we eat. This treatment is being recommended by many and is well worth a try. The peas will supposedly encourage the destruction of impactions.
Conclusion
Remember, always keep your water condition good and feed sparingly. This will keep the occurrences of swim bladder diseases to a minimum.