I've used frozen peas and popped them in a for a quick blend in the blender, then I just droped the little chunks into the aquarium. Fish loved it... I read somewhere that peas are very good for the digestive system...
Tried Zucchini but ended up with a mess, too soft. Tried cucumber, it was much better and the fish devoured it..
Also tried romaine lettace but the fish didn't know what to make of it.