Hi,
When I am preparing my own meals, I will save bits of cooked brocolli, cauliflower, brussels sprouts, etc. and feed them to the fish. I just break them up into tiny pieces and drop them in the tank. With the brocolli and cauliflower, I feed the florets rather than the stems. The fish just gobble them up. The same with bananas. I just break some tiny pieces off the banana and drop them in. The fish love them. Also, I keep a box of frozen, chopped spinach in the freezer. When I don't have veggies prepared, I can chop a bit off of the block and thaw it out in warm water. Just tear this up into bit sized pieces and feed.
You can also supplement your fish's diet with bits a fish, shrimp or crab