All was fine this morning before work, and i hadn't noticed anything odd right away after work. While eating dinner i noticed the tank wasn't active like normal and upon closer inspection i felt sick to my stomach. I spotted a dead barb. Then another, and as i got more worried, we spotted another, and also that the pleco was dead on the bottom!
I instantly thought something must have went off with the water but as i checked the thermometer it was off the charts.
The water was actually HOT to the touch.
The digital read over 98 degrees and i noticed that the heater light was still on, it was still heating up! I quickly unplugged it, and as i was lowering the water level to get some surface agitation we watched with broken hearts another barb struggle and then die. The remaining barbs were gasping for breath at the surface, as with the heat the O2 levels would be very low, not to mention just deadly hot

**So far** thankfully as of now as far as i know all 5 of my Dario are still alive. Unbelievable but maybe the only bright light so far in all this. I have now only 3 Tiger Barbs left from a school of 9, a dead bristlenose pleco, and the RTS is somehow still going strong.
The heater somehow got stuck on and heating up the tank nonstop. It's a normally very reliable Eheim Jaeger, everyone i know and all the stores i visit seem to have them without exception and all swear by them..
Sadly we were unlucky enough to get what appears to have been a defective one. I have no clue what could have caused this, nothing was turned up on it, nothing was touched and everything was keeping steady and healthy and strong for several months. I don't know what i can do to prevent this in the future but it really sucks for lack of a better term.
As far as fixing the situation, i didn't want to put what was left into shock. So i removed the heater quickly, it actually steamed when i pulled it out, lowered the water level to get surface agitation from the spraybar. I then did about a 50% water change to get some of that hot water out and some cooler water slowly in, to lower the temp very gradually some. I'm hoping i can save what's left, it's a few hours later and it's sitting at around 89 now and slowly dropping.
What is left alive seems to be fine for the most part. Is there any other course of action i should take? Also is the tank fine for a day or so without a heater as it cools down? I'm just worried sick about what's left and want to make sure we're doing the right things. I assume dropping the temp too quickly could lead to all kinds of problems and possible shock etc
We rushed to the LFS before it closed for a replacement heater, and upon hearing our story they have offered to reimburse us for the new heater even thought it was well past the 3 months warranty period in store. They were pretty shocked as well as they have never had a problem with the many of those heaters they use. A nice gesture i thought.
Anyways sorry about the huge post but we're pretty concerned.