U have a lot of surface area with a 175gal. I understand why u lose so much. You're AC must be working overtime to rid that much humidity.
I have a 55Gal freshwater that never needs topping off. It also depends on what sort of covering u have on top. If u have 1 at all. My Bio 14 loses about 1/4 Gal. a week due to evaporation. If I'm gonna be gone a few days, I just raise it above the Max. level line a little. Doesn't hurt anything