Les,
The first thing that I would do is try to get the nitrate down to under 20ppm. The pitting in their faces is typically a result of high nitrate. Contrary to what a lot of people say, a high nitrate concentration will poison fresh water fish. I keep mine under 20ppm. Ideally 10ppm.
The only way to do that is with water changes.
Do you have a way to pre-condition water?
http://happycichlids.com/Tank_Setup_and_Maintenance/Water_Conditioning_and_Prefiltering.html
To do more than 25% a day you'll need to go the High Quality method and that will require some infrastructure.
Once you stabilize the fish and the nitrate a fish that large in a tank that small, you'll probably have to change 50% of the water at least 2 or 3 times a week. He should probably be in a 75 gallon or larger.
Are you sure that your ammonia is 0 - .5? It should be 0. /Any/ ammonia is bad and indicates you don't have enough biological filtration media.
The other thing that I would do is put him on an antibiotic for external infections and add 1 tbs salt per 5 gallons. Pre-mix it in a separate container and add the total amount of salt slowly over a whole day. This helps to disturb the bacteria.
Make sure if you are changing water that it has the same salinity or you will shock the fish.
You could also try a salt bath just prior to starting the antibiotics, that is if the fish is strong enough. If so, it will help to kill the bacteria.
http://happycichlids.com/Freshwater_Disease_Identification_and_Treatement/Salt_Baths.html
I hope this helps!
-- Ryan Chapin
HappyCichlids.com
