There have been reports of otocinclus attaching themselves to broad, slow movig fish such as Angelfish, severums, and Discus but IMHO these instances are rare provided the oto's have a sufficent amount of algae and veggie supplements. Algae is the oto's primary food and in the absence of this It is possible that they would seek the slime coat of fishes. Planted tanks also provide the oto's with food as they will eat dead or dying leaves of plant's but do no damage to the plants otherwise. I would not be afraid of oto's with discus as long as the oto's have plenty of their favorite food. I WOULD stay clear of larger plecos and chinese algae eaters. These fish quickly lose interest in algae and find fish food more to their liking and the chinese algae eater would be more apt to harrass the discus. Oto's probably would not appreciate temps. of most discus tanks but many keep them in such temps.