I have found that it takes about a 10 gallons densely planted to sustain 1 oto without frequent supplimental feeding. Even then you may need to feed it about once a month to add to what it is getting from the tank surfaces. Let it's activity level guide you in when you need to feed it. A well fed oto is doing something a majority of the time, if it is stationary more than half the time you may need to feed it a little.