What sort of fish are you feeding?
Fish that need plant matter in their diet can be fed a variety of fruits and veges. My fish would eat pumpkin, zucchini, cucumber, apple, one rainbowfish ate banana, rock melon (cantaloupe for the US folk), dark green leafy veges like spinach and silverbeet.
Just make sure they are free of chemicals and have been rinsed well.
You can blanch the food items to soften them up. It also helps the fish digest them easier. Fish don't naturally eat terrestrial based plant matter so they don't digest it as well, and don't get as much nutrition out of it compared to aquatic plant matter.
Do not feed them onion or onion relatives (garlic, spring onion, shallots, leeks), and no potatoe.
Marine algae (available from Asian supermarkets and some normal supermarkets now, can be fed to fish.
You can put smooth rocks/ pebbles in a bucket of water out in the sun and when it is covered in algae, put the rock in the tank for the fish to graze on.