Otos should last for several years. If they aren't, most likely either a) the stock went too long without a meal and lost the bacteria in their gut needed to digest food, or b) they starved to death for lack of adequate algae. I have 5 in my 55, and they are all fat and healthy, been there for about a year now. But, I seldom see all 5 at one time, since they easily hide on the underneath of large leaves and within denser plantings. I agree with cdawson--they are not really appropriate for unplanted tanks.