Did not go thoroughly through all the posts. IMHO it's not the parameters but a bacteria issue, rather a lack of. Do you carbon dose? If not you should.TM Bactobalance is best for you as it has traces of phosphates. Your tank is young, and your bacterial diversity may be insufficient for your corals. Give it a boost with the carbon dosing. Chaeto is great, but it's a trace element sink so you should consider adding trace elements regularly. I would not do water changes as they are bad for stability.