It depends on the fish, diets vary widely. Most any veggie is good for fish that will eat it, zuccini is a top one for plecs, along with most any other variety of squash. Spinach, carrots, about the only one that I can think of that is not such a good idea is corn, as it is tough to digest.
For fish that like a more meaty diet most any fresh uncooked seafood is good.
Processed foods such as bread are a no go, too much bulk for the nutrition they receive, as well as being loaded with preservatives.