My 1400l mixed reef was doing great for the last 3 years, but now my soft corals (GSP, zoas, blastos, acans, and even xenia are stalled or dying, having previously grown like weeds). I have a very large red plating montipora which is growing fine, along with blue stylo, digitata, chalices, millepora. My euphyllia are alive but stalled. Using Triton with chaeto, plus a 10% weekly continuous auto change, plus kalk stirrer. Nutrients fairly balanced between fish and coral, but I run a skimmer and GFO too. Alk consumption is as high as its ever been, so the SPS is thriving. I had some instability a few months back in both salinity and kH, but now resolved, and my parameters are stable for the last two months (440Ca, 1350Mg, 8dkH, 35ppt, 20ppm Nitrate, 0.1 phosphate). No change in lighting or flow or feeding or temperature when softies were flying (although zoas and blastos were always a bit stop start in the tank). Fish are all happy. Water RO/DI is 0TDS. ICP test not showing anything out of the ordinary. All measurements from at least two sources, all probes calibrated correctly. I do have a 50mm/2" deep sandbed which I can't easily siphon clean as the corals have grown out. I also had an aptasia outbreak, and now what looks like green cyano on the sand. Could either of these upset the softies that much? Could the big red plating montipora be outcompeting all else? Or do I have old tank syndrome and if so, how would I fix it? I'm out of ideas, any suggestions very welcome!