Courgette is better than cucumber; more in the way of nutrients. I have read that otos will eat sushi nori but that it can make a bit of a mess in the tank. If you can cope with the mess it might be worth a try. And I've just found an article on Practical Fishkeeping's site that suggests blanched spinach.
Aha! I have just found a very good article on otos on wet web media, written by the fishkeeping guru Neale Monks.
Here The first part of the article deals with the various species with how to keep them in the second half. In addition to spinach and sushi nori, he says that wood in the tank is essential for otos. I know that some species of plec need to rasp at wood as part of their diet, and it seems that otos might be the same. Do you have any wood in your tank? If not, it might be a good idea to find a small piece and put it in straight away even at the risk of turning the water brown. Wash it first to remove any dust etc!
And for those of you who have never heard of zucchini as mentioned in the wet web media article, it is American for courgette