I suspect that you are seeing Swim Bladder Disease which affects a fish's swim bladder. This is the organ that helps in maintaining a fish's ability to balance and swim in a normal position. When it becomes blocked up or too full of air it stops functioning properly.
This can be a result of water quality, over feeding dry foods or a Bacterial infection. Poor water quality can play a major issue contributing to the Swim Bladder problem. The first step to helping is to check your water quality. The Ammonia should be 0, Nitrites 0 and the Nitrates 20 ppm or less. You should have an API test kit to check the levels.
We can try some first aid to help the fish.
1) First do a 3/4 water change and be sure to add some de-chlorinator or use bottle Spring water for replacement.
2) Next please feed a couple of frozen peas in which the shell has been removed. You will need to be sure you crush the peas for the fish. This helps the stool to pass and will sometimes help to relieve the problem.
3) I also suggest using aquarium salt at 1 teaspoon of the salt per gallon of water. The Salt helps to draw out fluids that can cause swelling in the swim bladder disease.
These are a couple of links that give you information about swim bladder disease:
If the fish is in with other fish it may be wise to set up a hospital tank which will make it easier to treat the fish.
There are some other methods of treating swim bladder which include reducing the amount of water in the tank which helps to decompress the swim bladder, but I would only use that as a last resort.
This link will help you locate Fish Vets if the treatment does not help: http://www.koivet.com/a_fishvets.html
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