I agree, Tang will need a bigger tank soon, all part of the plan
Water changes get done every Sunday, 3 gallons of RO water at a time. I use Instant Ocean Salt and I vaccum the 1-1.5 inch sand bed. I do have a lot of evaporation as well, I add RO water every evening to the sump, total is 3-5 gallons per week. I have 65-70Lbs of Live Rock and no mechanical filtration. My tank is drilled in the right upper back panel. The piping is setup so the surface is skimmed and the return line (quietone 4000) is moving the surface. I also use 1 other powerhead on the opposite side. Skimming with a Seaclone 150. Have to play with it to keep it going once or twice a day. I get 3/4's of a cup a week and it stinks, real bad. Very thick and black. I use to have a sponge on the end of my overflow in the sump, never cleaned much out of it so I slowly stopped putting it back in
Temperature was always 82 to 84. I lowered it to 76-78 and I have changed my feeding patterns.
Never tested for Magnesium though!