Yes, fish get sick too. And sometimes, fish don’t get along. You may wake up one morning and look in your fish tank and see one with a fin half bitten off, or just swimming abnormally. Believe it or not, there is something you can do for a sick or injured fish.
You will need a bucket or a small spare aquarium for the job. Make sure you start with temperature adjusted and treated water. The last thing a struggling fish needs is a temperature shock or untreated tap water.
Follow the directions for adding aquarium salt to complete the water conditioning process. Also, use a clean filter for the bucket or aquarium. Your fish may need to use a fish antibiotic solution like fish mox or fish flox forte. Consider using a natural treatment like Melafix, which is made from tealeaves.
You can use 2-3 times the recommended amount of the antibacterial solution and taper it down as your fish begins to recover. Try not feeding your fish for a couple days. Digesting takes a great deal of energy, detracting from the healing process.
If your fish really begins to struggle, and if it is a fish you really care about, you can help it around the water for a while by gently moving him through the water with your hands. This will give your fish a break from trying to swim, while keeping it oxygenated. This process has helped save some fish, but obviously it is a time-consuming task and is only worthwhile for someone who is really invested in saving a fish.
You can soak fish food pellets in some olive oil and an antibacterial gel or powder, designed for that purpose. Just make sure you dry off the pellets with a napkin or other absorbent material, before you drop it into the water.
As long as your fish is isolated for healing, change the water every day. Make sure, of course, to provide it with properly heated and treated water. Be very gentle during the water changes, to avoid stressing the fish. Once the fish is acting normally again, and any visible wounds are healed, you can return it to its original tank.