It could be something other that biopellets causing this. I had the same type situation occur in my 210 gallon SPS reef just over a year ago. Everything was fine, then I started experiencing everything from STN, RTN, and sometimes even entire Acro colony loss overnight. Tested, retested, and had others test, looked for pests none, etc, just like you. Couldn't blame it on anything. Finally, I had just resigned myself to having a softy tank when I decided that it was a good time to upgrade to a 300 gallon. Didn't have much to lose since the SPS were headed south anyways. I went from an AGA 210 RR with a reef wall type setup to a Marineland DD 300 with an open style aquascape, and also went from 4x Tunze 6105s to 4x MP40wES pumps and the STN/RTN/Total loss stopped in its tracks. It was like flipping a switch. I have since concluded that when all my Acros were little frags and small colonies, the Tunzes and the reef wall Aqauscape were sufficient regarding flow, but as my colonies matured and got some size on them, the reef wall and the Tunzes just did not provide the flow characteristics the corals needed. Some of my acros that started out as 1/2" frags two years ago are larger than dinner plates now, and I can't help but think that larger acro colonies can radically restrict water movement as they grow. A reef wall aquascape further restricts water movement, and I don't think the 6105s were giving me the undertow and type of water movement the Vortechs do. I personally believe the corals were either starving or smothering (or both) from lack of flow in my 210 gallon tank.
I have since changed out two of the 40wES pumps to MP60s (one 60 on left/right side and two 40s on the back).
So you might want to look at your aquascape and your internal water movement as a possible cause of your troubles. Sounds like you have some large colonies as I do. Also, I maintain an alkalinity of 10-11 in my 300 and I have not had any issues with acroporas because of it. I have not heard of high KH being necessarily detrimental to SPS.