Heater stuck - any preventative measure for the fish??

Well it happened  ....  I bought 3 fish at MACNA - 2 clown and an anthias.  Set them up in my quarantine tank.  Today the temps were lower so I added a new heater.  Didn't think about it, but about 3:30 noticed that the anthias was breathing hard.  Then noticed that the thermometer wasn't registering.  Got out the coralife temp, put it in and the temp was 90.  I know that its the huge temp swings that can really wreak havoc - but at that temp I thought there was no other choice but to lower it fairly quickly.  Added some ice in a sealed bag and brought the temp down to 84 over the period of 1 1/2 hrs.  Also used an air pump to pump some oxygen into the tank.  The anthias however didn't make it - its already dead.  The clowns look ok - they aren't gasping or anything.
Anything else I should do to help the clowns over the stress they've experienced? - i.e. - anything I might add - selcon, vitamins, garlic......  Obviously I'm going to watch the heater closer next time I turn it on.  Should I let the temp gradually work down now to 78 or should I use some more ice to lower it some more????

 

Thanks for the responses.  I shut the lights off yesterday as soon as I noticed the heat problem.  Today, the clown fish are doing fine.  Let's hope they stay that way!  I added some vitamins this morning.  I didn't try to even feed them yesterday.  I'll try that a little later today.  They haven't been too interested in the food I've tried - mysis shrimp, prime reef flake, brine shrimp (gel - from MACNA), or brine shrimp flake.  They may have eaten a little cyclopese.  I've tried adding a drop of garlic also.  I think I'll go and get some New Life Spectrum small granules and try that.  My first clowns really liked that years ago.
As for the heater - I'm sad to say it was an operator error.  It was a new Eheim jager heater (I've used them before).  I just set it and put it in the tank and didn't think to watch it closely.  These new heaters have a calibration ring on them - and that had gotten out of wack - so the temp was set to 90 plus!  A sorry mistake on my part for the anthias!