Why the hell do my sps keep dying

[Snappy]

You say they die from the bottom up. How quickly? Is it RTN or STN? In my experience if corals are dying from the bottom up they are either getting too little flow or too little light or both. I can assure you that in a 75 gallon tank you are not even close to the flow you need for sps with your current power heads. IMO those KH1's are barely enough for a 20 gal nano. As an example I have 2 sieo 820's & a Tunze nano stream (1200 gal hour) in a 40 gal frag tank.
It's a big jump going from a FOWLR or Soft coral reef to sps because the requirements are very different and flow & lighting are key in keeping sps healthy.
Also what brand of salt do you use? Have you tested the new water mix and if so what is it at?