High levels of nitrates can effect fish immune system coupled with the ammonia are probably the cause daily water changes and add a double dose of prime to detoxify any ammonia or nitrites till it gets to zero if you can get a 5 gallon tank or you can use a plastic tote to Qarantine him the most effective treatment I have found for Dropsy is kanaplex dose the tank to treat any possible bacterial infections metroplex in food feeding a small amount twice a day for 3 weeks treats aeromonas bacteria a common cause of Dropsy and aquarium salt 1 table spoon for 2 gallons to help reduce the fluid buildup or you can do Epsom salt baths 1 table spoon for 2 gallons for no more than 15 minutes @ChanMan