Many fishes love veggies and fruits, especially plecos. Watermelon is the true treat to my bristlenoses and gold spot pleco. It's not odd that other fish nibble them, too.
Courgette (fast cooking for softing) is the best, cucumber is good but has little nutrition. Lettuce, quickly a bit cooked carrots are good too, and many people feed peas (with their skin of) for fish with stomach problems.
They are good little treats to give every now and then when you prepare them at first, rinsing them good, or giving them a bath in a boiling water if they are hard, and removing the skin from cucumber and courgette.