I have almost 200 Dempsey fry,a little over 3 weeks old in a 20 gallon high tank.Tank is filtered with a AC70 and a old Penguin 200 with a bio wheel.Both have sponges over the intake tubes.I change 50% of the water every other day.Question is, how long do you think I can keep them in the 20 gallon before I have to set up an old 55 gallon I have and put them in there?