I'd recommend getting your salinity checked or purchase a refractometer if you haven't already. Once you get things dialed in, it won't be something that you need to use every day, but it is nice to have. (I'd skip the plastic hydrometers...they are fairly inaccurate.)
It looks like you have a lot of salt spray in your stand/sump. Try to figure out how to minimize that. Cover the area where the drain is (although it looks like it could already be covered). Perhaps slow the water flow through your drain and sump by using a valve on the discharge side of the pump. As far as evaporation, it isn't uncommon for a tank your size to go through 2 gallons of top off (fresh) water per day. Things that effect this are how much water movement you have, how fast water flows through your sump, temperature of the tank, and humidity levels.
Evaporation isn't a bad thing...and can be helpful in the summer to keep your tank cool. But it is something that requires daily attention. Many people get an auto top off so they don't have to deal with it daily. But top offs also can cause their own problems.
Many people use tap water. The Tropicorium successfully uses tap water. Its always a little more of a gamble than using RO water simply because we don't know what is in it. Public water supplies often add phosphates, chlorine and fluoride to the water, which can cause problems for reef tanks. The advantage to RO water is, it provides a known starting point (hopefully 0) for you to mix your water and eliminate unknown variables. By looking at your tank pics, it doesn't appear that you have algae issues which I would immediately look at the water source in your case.
To me it looks like you are on the right path: You are here asking questions. You don't have a ton of livestock yet which is good. You already discovered that you had an issue with your temperature. I think once you get on the right track with your salinity you will see your polyps open again. In this hobby it seems that good things don't happen fast. Take your time and figure it out one step at a time and feel free to ask here. That is what we are here for.