[marie]

Maybe try taking the acro out (if it's not too far gone) and puting it into a bucket of fresh mixed salt water with a powerhead, heater and a light of some kind on top

I don't think a person can measure success by how many mixed tanks work but by measuring how many mixed tanks fail

To be honest I don't think it could be anything else. You've measured for everything else and everything is where it should be. Therefore it is something you can't measure for and with all the other inhabitants being healthy and growing it isn't something toxic (to them)