Open Top Sump/Ref Lose a Lot of Water Due to Evap?

Hey guys! Just wondering, I am about to start my 40gal sump/ref build for my 100 gal, and I notice a lot of you guys have an open top on your sumps... don't you lose a lot of water due to evaporation that way? I only ask because I have a glass canopy on my 100gal DT and I NEVER lose water but when I accidentally left it open for a day a long time ago (before I had my wrasses) I lost a bit of water... wouldn't an acrylic canopy, cut to accommodate the skimmer, be better as to minimize water loss and fluctuating salinity levels? Let me know what you think! Thanks!