My otocinclus have been hiding much of the time lately which is unusual....at least for the babies that were born and grew up in this tank. The ones I bought at Petsmart have always hidden more. I checked nitrates which were under 10, so I figured that wasn't the problem. Then I recalled a few times that when I had an airstone running they were much more active. In fact, when I had them in a 10 gallon with a betta and had an airstone running, that's when they bred for me. So, I dropped an airstone in yesterday afternoon as an experiment to see if they came out to play more with it running. Sure enough, 4 of my 8 are out and about today as compared to yesterday when they were hiding in the cave much of the time. So if you have otos and aren't seeing them much you might try adding extra oxygenation. Mine seem to appreciate it.