Just try different kinds of food see if anything he like. Yeah I done loads research before I got as never had fish before and still do when see something odd so make sure fish are happy and healthy. Don't no what could course the evaporating as seems a lot but depends on how big your tank is and if u got hood/lid but that's just a guess as dont really lose much water
I have a 110 gal and loose 1 gal per day to evap. ATO keeps things in check. My ATO resevior is refilled every 5 days and its 5 gals.
A standard 75 has the same footprint as the 110 and would have very similar evap.