I agree with russ haha...
Few things.
1. there is no such thing as you cannot Install an Rodi. There are plenty of adapters and fitting to make an ro unit hook up. And if you have low pressure, get a booster pump. Don't use tap water. I use to ALWAYS use tap water. what an idiot I was back then. I use to think ohh it's no big deal, my tank looks fine, yadda yadda. Ehh wrong. HUgE difference when using Rodi water. I will never ever ever ever use tap again. I always make sure I have a garbage can on hand full of Rodi water.
2. Obviously your tank will evaporate. Common sense. I have a little 46 gallon and I'm constantly adding 2 gallons ever 2 or so days. All depends on your surroundings and environment. If you feel like your more of the lazy person and don't feel like filling it every few days, go get a
auto top off unit. I like the new gravity fed one by toms aquarium. Besides that, everyone deals with it.
3. Do your self a favor... go out to your local hummer dealership. Ask for a test drive of the largest bad ass truck they have. Drive it to your house. Run into your house and grab your video recorder and your coralife hydrometer. Place your coralife hydrometer in the road, and your video recorder a few feet from the hydrometer. Press record on your recorder, Get into the hummer, and run over the hydrometer. Do it again, and again, and maybe a nice burnout on it. Then grab your recorder and bring the truck back to the local hummer dealership. Then go home and upload the video on your computer and go on this site and make a thread called "good bye hydrometer!" and paste your video on that thread. After we all watch it, and laugh, then pat you on the back, go grab your phone and call russ at saltwater critters and tell him your on your way over to
BUY A REFRACTOMETER
Refractometer...
http://saltwatercritters.com/inc/sdetail/967
Tom aquarium auto fill system...
http://saltwatercritters.com/inc/sdetail/4871
Rodi unit...
http://www.airwaterice.com/product/1TYPHOON/Typhoon-5-stage-RODI-Reefkeeper-75-GPD.html
Booster Pump...
http://www.saltwatercritters.com/inc/sdetail/3520
Hope this helped...