Thanks everyone. All 18 tanks have been doing great until this last month. Three tanks (over 4 weeks) have done this with only 20 / 25% water changes. Yellow lab adults saved by removing from tank once fry instantly died. Red-top zebra cichlids qty 50, only saved 4. So when my Midas started gasping for air, I just freaked. My local water dept swears to me they only add chlorine and nothing else. I let my Fresh water sit at least 36 hours with air stone and heater. Triple checked temp it was less than 3\10's of 1degree dif. I should not need to add water conditioner, but apparently should. Other tank WC where fine. Could it be the colder outdoor temp keeping more chlorine in the water lines? That's the reason my water dept gave.
Will be on the lookout for that huge bottle of Safe.