Originally Posted by mr_X
You do not want to prevent evaporation. You are better off topping off than hindering gas exchange.
Boo... My mom, a non-fishtank type person will be tending my aquariums. But real basic stuff--feedings, checking to make sure equipment is running & topping off the big tank. We won't be gone long enough to interfere w/ the PWC
schedules. I didn't want her to have to deal w/ knowing how much to put in & IDK an average of how much evaps out. I will just have to draw lines for her & say "Fill 'er to there..." It will be fine but I worry a lot regardless.