I have a 120 that's currently open top and sump. I lose about 2 gallons a day. It might go up in the summer or if you put Halides over it. 1g per day seems just fine.
Yep the 180 does proably 2 gallons a day and that's now in summer might be a lil more! Now in the cold days you can see the condinsation on the inside of the windows.
My 90 is about 1-1.5 with canopy. The 30 has a glass top and every day I loft the glass and drain the water back into the tank. Salt doesn't get on the glass top.
I had an issue with replacing that much water too and ended up buying glass tops. It didn;t reduce the evaporation very much but I did have some par readings done and it reduced the par by a good 40%. After replacing my glass lid with netting I have found that my growth has increased and everything looks happier. I did have an initial algae bloom which passed after about a week. Just stay open top, you really need the oxygen exchange. I'm sure our elevation already leaves us challenged as it is.