Yeah when you use fans to cool off the water, evaporation will increase quite a bit, hence why I installed the ATU. My pump is only a little 200l/h pump. it pushes the water easily up. Remember, it only needs to push a small amount, so even if the pump struggles a bit, it should be fine. You can add a larger pump to suit your needs (depending on how low your reservoir is placed in relation to the tank - mine is the same level, hence the small pump), it'll only pump for a small burst to get the water level right again. Space is an issue in the TL550's, so get the smallest unit you can find.
With regards to the Ph, I used tap water with SeaChem's Prime for the initial startup of the tank, which wasn't ideal, seeing as the prime never really takes out all the crap of tap water. I did a water change with RO thereafter, so I think it must be the reason for the drop. I also used Microbe-Lift's Special Blend to get the bacteria going in the tank.
Salinity is stable at 1.024, right at the mark for lps and sps.