lindsay you could try salt baths, just disolve the right amount to aquarium salt in say 1 gallon of water in a bucket, add fish for 15 or 20 mins, repeat daily, if he improveds reduce to every other day then once a week, you can do this as a tonic, is says on the packet, for your other fish too, put them in first together for 10 mins,
I think only weekly if they seem fine, it will ward of any infection they may be harboring from the other fish, then take them out and add the sick fish.
The dosage is on the pack, a couple of table spoons per pint or something, i expect its slightly different depending on which brand you purchase.