Hey Kevin. I had a 125 with an open top and canister filters and I had a 125 with glass lids and a sump. The 125 with the lids and the "open" sump, had far more evaporation that the 125 with no lids (probably about twice as much). I think the reason is because the sump loses a lot of water through evaporation because of the constant moving water. This is just my experience. HTH.