[arctictwist]

So i understand that if you have a sump you can do top offs and water changes in there. But what i dont understand is how having the sump keeps the water level in your DT stable. If heat causes the evap, why would it not evap out of your display and if the pump flows at the same rate, how does adding top off help that?

[Ganrin]

The return pump and your overflow, either hob or drilled somehow, keep the level of the display tank constant. Evaporation loss lowers the water level in the sump. In a tank with no sump all evaporation comes from dt and dt water level drops, in a tank with sump both sump and dt evaporate, but sump replenishes water level in dt.