Every fish in my 60 gallon (20g sump) has died besides my melanarus wrasse. I had a pair of pink skunk clowns, mandarin goby, lemon peel angel, coral beauty, powder tang, clown tang (both tangs were under 4” and would have been transferred to a 200g that is being converted to salt now), squareback anthias, red linckia star, and blue linckia star. I was originally having issues with my zoanthids opening, so I overfed for a day or two to raise my nitrates and phosphates, but made sure to monitor the 0 ppm ammonia and 6-8ppm nitrate range as well as keeping the ph at 8.3 and phosphates at 0.05. I’ve added Red Sea magnesium booster and the levels are now at around 1250 (min) 1350 (max) as well as a ph and calcium (450 which seems a bit high?) buffer periodically to keep them in check. The zoanthids opened for a few days and everything seemed to stay in check. While overfeeding, I turned the return pump flow down to about 650 gph for a night with my wave pump on. The next day, I found my powder blue tang and lemon peel dead, as well as my linckia stars pushing out their organs and my zoas closed. I automatically turned my return back to 1050 gph. The next day the linckia’s both died as well as the anthias. I continued to check the parameters and everything was still in check. Yesterday, my coral beauty, pink skunk, and clown tang died, so I decided to do a 50% water change, change the media, and check the parameters. Today, I came home and my other pink skunk and mandarin goby died and my Duncan coral completely retracted. I then did another 50% water change (ro/di, 0 tds with 35ppt salinity). I am testing with all salifert test kits and adding oxygen with an air stone. I’m assuming a chain reaction happened where the lowering of flow killed my stars, poisoning the water, and killing my fish, but I’m not sure. Please help. I’ve lost around $500 in fish and don’t want to lose my corals now.