They really do much better in twos or threes but once established they will do okay with one. He was probably either on his way out before or had gotten horrid care before you got him. They are notoriously horribly mistreated by their captors (they are wild caught by flooding the water with cyanide) and half die before they ever hit the stores at least. Then most pet stores insist on feeding the fish flake food which they do not eat well so they are basically starved as they also wipe the tanks down and do not have algae in the tanks for them. So another large percentage die of starvation due to inadequate or the wrong food. The real problem is that if they are purchased after being starved for too long, the bacteria that resides in their stomachs to digest the food they do like to eat may have died off due to starvation at the store and you may not be aware of this. If this has happened, you can feed them well and they can eat until they look like they will pop and it will do them no good as the food cannot digest and they will still starve to death. They can appear totally healthy and still die of starvation. the only signs to look for in a totally healthy oto are:
Rounded bellies but not totally bloated like they are going to explode
White colored bellies (red streaks or dark red blotches usually indicate infections) some pale pink is okay though.
Eyes that do not have a glazed over appearance.
That is the best I can tell you other than to visit the site at:
http://www.otocinclus.com It is their own site and the information gives you an idea of just how hard these little guys have really had it.
Rose
I do sympathize with you on the loss of your fish. I have owned MANY otos in my fish keeping and only have 3 left but they are my oldest fish and so darling I just love them.