Losing 5 gallons a day do you ever have to do water changes? 5 out, 5 in daily seems like your water would be fresh all the time.
Yes! When water evaporates it leaves all of the solids behind in the tank. The only think that leaves the tank is H2O. Salt, CA, Fish Poop, etc. all stay in the tank water. This is why it's so important to keep you water level constant.

My 120 needs about 2 gallons a day. I'd say the MH lighting and sump cause most of the evap to happen. One of the best investments I purchased was an osmolator. I never come home to a low tank any more.