The first step towards dealing with these problems is to first check on the water quality. Proliferation of bacteria and parasites is often identified to be the root cause leading to the disease and affected fish will look lethargic and become listless with reduced food appetite.
Fish with fin rot disease
Other than maintaining perfect water condition, there are other approaches one can use to deal with it. One of them is to raise the salt concentration to 0.5% while at the same time apply some antibiotics available from local fish shop. Another form of treatment is to purchase medicated flake food and feed to the fish. If everything is done perfectly and provided that the symptoms are still in the early stage, chances for the fish to recover is very high.
Some posts covering similar topics:
» Tips to successful aquarium fish keeping
» Common mistakes made by aquarium owners
» Other fish disease - Ich or white spot