Some pet shops sell fish vitamin supplements. If you can't get them use a bird or reptile vitamin supplement. Try to get one in a dry powder form and sprinkle it over some frozen (but defrosted) food like bloodworms and feed that to the fish once a day. Give vitamins 6 days a week and none on the 7th day unless it's a fish specific vitamin supplement.
You can cut up soft leaf aquatic plants like Ambulia and feed that to fish, algae is also useful and you can scrape it off the glass and feed it to the fish. Duckweed can be grown on the surface of the tank and when it is grown under coverglass, it develops a soft leaf and can be cut up and fed to fish.
Feeding the fish a varied diet including dry, frozen (but defrosted), and live foods gives them the best chance of getting all the nutrients they need.