You can try feeding canned, preferably no salt peas, the inside mush type parts, not the shell of the circle Pea. You can steam/cook your own from fresh or frozen as well.
A squeeze of garlic juice onto the food can possibly help with appetite.
With hold food for a day then slowly feed the inside parts of the peas, as the fish eat them add more - remove uneaten food after a couple hours.
What is the pH? Do you know the TDS