welcome to R2R, nice first tank.
I would move the rock in the corner to the white one in the middle and build a small cave or passageway, it'll give your fish a place to hide and they will be able to swim through, I would connect it from the white middle one to the purple rock.. similar to an archway. This will get the rocks away from the glass, as stated above its good to allow room for the magnetic cleaner to pass by.
If you don't want to get a refractometer, I would suggest getting a Hanna Salinity tester with the temperature gauge. I have one and it works good.
If you want to hide the tubes and wires behind it, you can get decorated backgrounds at almost any fish store or online instead of trying to paint it while there's fish, water and corals in the tank... probably not a good idea.