I lose about 5-7 gallons a week in my 90 but I have to keep the top partially uncovered due to heat buildup. I live in Fl and tightly covered my tank can get up to 83-85 degrees due to heat from my four filters. Makes my cichlids nuts. So I go uncovered and I can keep the temp around 78-80. I have to add water everyday or so, but I makes for a much more peaceful tank. Something to think about depending on the amount of filtration you use and ambient tempin your room
its normal. i lose about an inch every 3 days. glass top helps but just a lil unless theres no roles on the top otherwise if theres a gap it will vaporize as well but not as much as open tops.