1) Take a look at the ailing specimen, and take note of what is different from normal.
2) Check to make sure that all environmental conditions are in order, because the number one cause of stress, ailing behavior, disease, and death in aquaria are environmental problems. Test the pH, ammonia, nitrite, and nitrate levels, and if possible, the oxygen content. If these are in order, make sure that no other toxins such as paint fumes, pesticides, or cleaning agents could have entered the tank. If the answer is no again, make sure all accessories are operating properly (heater, air pumps, filter, etc.). Try to see if another tank mate may have been on the aggressive. Run down the list of the fish's requirements to see that they are all in order. Remember that tank layout and dietary deficiencies also can affect the fish's health.
If you find that one these conditions is off, then take steps to fix it. If the water chemistry is the problem, make water changes until the problem is fixed. If everything checks out, it is time to move on to the next step.
3) See that the hospital tank is in order, and set up two buckets full of water (The water should match the main aquarium's water conditions and be dechlorinated.). The buckets should be at least 2.5 gallons (9.5 L), though 10 gallon (38 L) tanks are preferable for large fish. Be sure that the containers are well-aerated.
4) Move the ailing fish by net to the first bucket. Add 3/8 TSP of salt per gallon (0.5 g/L) every two minutes for 10 minutes. Using the net, move the fish to the next bucket of fresh water for 30-45 minutes. After 15-20 minutes, add 3/8 TSP of salt per gallon (0.5 g/L) again.
5). Net the fish and move it into the hospital tank. Be sure to change the water daily (30-50%). After a day, move the fish into a shallow, clear bowl. Examine the fish for parasites and evidence of disease. If necessary, use a magnifying glass.
Treat the water accordingly.
6) Keep the fish isolated for at least 10 days.
Specific Diseases that Affect Tropical Freshwater Fish