I have a indoor 3 feet tank, running on 2 PC fan alone! running only in the day , when lights are on; average temp day is 26- 28, at nite w/o fans is 24-26) I place one on each end, with one inverted, so air goes in and gets carried out other end. Place nearer your lights source, as the heat is the main problem.Evaporation rate is very high, 1.5 litres a day!