after noticing the same oto had been underneath a plant for half the day sucking on gravel, i moved the plant a little, he didn't swim off, so i kinda pulled him on the tail and he was dead..not sure what did it..the other fish are doing fine
i heard that ottos need a VERY mature tank..i might get them a YEAR after i started in my 55 gallon tank..which is august so ur tank is reli new..so maybe that's it i dunno