Evaporation!

hmm, im averaging about 2 gallons a day atm, your doing fine.

 

Evaporation is annoying but it will help a lot in controlling your tank temp which is a big plus in warmer seasons,especially when using m/h lighting which adds heat.Keeping your tank topless will also give you much better gas exchange,another good reason for keeping it this way.I'm sure you'll work out an ATO before long as ot seems like the best way to go with a large tank.Reef Sparky recently posted a thread about his ATO that uses his ro/di directly that you may be interested in,his idea sounds pretty good.