How big is this tank? How large is the return section in your sump.
I have a 120g tank with a 55g frag tank plumbed into it and 3 different sumps (about 240g total) I have to top off 3+gallons per day.
When you have larger water volumes, high flow and several sumps/fuges you are going to have a lot of evaporation. I keep a dehumidifier near the tank so I can keep the humidity in check (during the summer).
One of the easiest solutions is to add an auto-top-off system to your tank. There are lots of brands out there or you can do a DIY ATO. I have one just haven't set it back up. They are really nice to have especially if you want to keep your SG
in check. Mine is a diy, simple, float switch, water and a pump. When I had it hooked up I kept a 20g tank next to my sump filled with RO
water and a maxi-jet pump with tubing over to my sump. When the float switch was activated (water level dropped) the maxi would come on and pump water into the sump til the float switch shut it off. Do a google search for auto top off and see what you come up with.