This may sound extreme, but I had a similarissue to you in my 120g tank when I first setup a year ago. I gotbrookynella, ich and all my fish dies within 3 months of setting up, Ifound out LFS gave me AEFW and I wasn’t aware and they also gave me the good oldred planaria flatworms as well. In my battle to kill the regular planariain my opinion I ended up dosing too much Levimsole into my tank to killflatworms I think. Because before I did that, my acros were growing fineeven despite AEFW, once I did that, Acros all RTN'd within days of being in thetank. Fish would die within days of being introduced. It was amajor mess in my dream tank.
So I basicallyleft tank to sit for 6 months with nothing but rock and water, left skimmergoing, etc. Once I got motivated in October timeframe last year, Idecided I would mix 100% new water and drain tank. So I mixed up 120G offresh saltwater, drained tank down completely into toilet with MJ1200 of allpumps over a 30 foot run. Took at least 4-6 hours to do. Then Ifilled it with 120G of water and that was another 25 foot run with a MJ1200, ittook a long **** time but eventually that night it was swapped out. I hada mini ammonia spike and within 3 days after swap, I introduced 3 SPS frags andthey are still growing and fully encrusted to this day. All acros havebounced back since I swapped water out. My tank is actually growing SPSlike weeds this past month it seems.
I highlyrecommend swapping entire water out. Let me know if you have anyquestions