Yes; cucumber is hardly worth bothering with as there's so little nutrition in it.
Tomato is very popular with all my plecs (and snails); the end slice is best so there's no seeds and the skin holds it together, but sweet potato, apple, courgette, broccoli (the stalks), melon, peas, carrot (needs a bit if cooking to soften; mine don't like it but I've heard some do) and lettuce are all things you can give.
I don't think I'd bother with potato; lot of starch in them and I feel they wouldn't be good for the water quality.