Goldfish / Aquarium Fish and Swim Bladder Disease
I first found out about Swim Bladder Disease (or flipover) about two years ago when my sons favorite goldfish appeared to have suddenly died. My son, distraught and crying thought that his goldfish, Darth Vader, was floating upside down and dead. I took my finger and nudged him a bit to make sure and sure enough Darth Vader swam off. It was then that I had to research so I could see what was going on and to give an explanation to my son whichever way Vader's life was going.
After some research I cam to find out that his goldfish had a common disease for aquarium fish called Swim Bladder Disease (or Disorder). The swim bladder in a fish is and internal organ that is filled with gas that allows the fish to control it's buoyancy without having to use excessive energy to swim. A fish with swim bladder disease can float to the top of the aquarium or sink to the bottom as well as floating nose down or tail up. In Darth Vader's case, he swims belly up.
There are a few things that you can do to attempt to resolve this for the fish. One common cure is to buy a package of frozen peas, then defrost them and take the skin off one or two of them. Stop feeding the fish for a few days so they are good and hungry then feed them the pea. This may take a few tries, but eventually they will eat it. Goldfish are constantly hungry so they will eventually take it in. The pea solution should work within a few hours, and could possibly be to resolving constipation.
Alternatively, you can check the water conditions for the fish to make sure they are proper and may notice that the problems get worse with water conditions. You can also pre-soak their food before giving it to them to prevent them from eating it to quickly at the top and taking in too much air while eating or getting air from the food itself. Another possible solution is to add aquarium salt which is good for fighting disease and boosting the fishes immune system.
Swim bladder disease can be a side effect of a bacterial infection, so it is definitely important to focus on the water conditions and prepare appropriately.
Try some of these and see if they help, in our case it definitely did. Best of luck to you and your pets.