So you are asking if you switched to T5's from a combo of MH and T5s would it cut down on evap? It may help a little but not drastically to take away the benefits of your MH. I run 4 48" NO bulbs over my FOWLR and have at least 2 gallons a day evap; with that said MH or not you are going to loose water to evaporation. Add a 10-20g tank to your system to use as a top off and you will be good to go.