Also try Courgette/Zucchini (which mine seem to prefer). . With Cucumber take the middle bit out, the seeds as IME it goes everywhere. Wash it, or you can to make it easier for them to munch on - blanch it or soak it in hot water for a couple of minutes. Weigh it down in the tank with whatever you can. I use plant weights.
My Cories don't touch it, they don't get a chance with greedy plecs. I usually put mine in before lights out and take whatever is left out in the morning.
