I couldn't grow SPS until I started chasing numbers. Especially the ALK number. Keep ALK a stable flat line on a graph and things will grow. IMO don't get too caught up in the nutrients and par etc because without steady alk and numbers you have absolutely nothing. Work out those details after stabilizing numbers. A lot of stuff can cause swings: too large of water change, not topping off the tank often enough, not dripping in alk etc etc etc it takes a lot of attention to detail but it's a fun challenge. Just pick wamas hardy pieces. Slimers, torts, staghorns are some fine selections to get confidence up. After stabilizing tank watch their colors that will help you to fine tune the details. After mastering the easier SPS buy the milli, tenuis, etchinata. 6dkh is too low.