In retrospect, triage was the way to go. I just finished the third water change at 20% a little while ago and just minutes later the snails, featherdusters, hermits, etc. began moving around. Some were dead. My noob mistake was thinking that taking the heater out of the water for only a few seconds would not cause it to crack --it did. (Yes, the copper was probably part of the problem but the water stank of burnt plastic.)
Also, in retrospect (for any chemical pollutant problem with the tank) the first water change should have been 40 or 50% instead of several 20% changes. Oh yeah --it was a Visi-therm.