Last week I noticed a few marks on my Corys , turned out they hat hurt themselves on a new ornament in my tank and the wounds had become infected with a fungal infection, some of the other fish were bothering them and luckily I had been cycling a tank in my daughters room ( to house a betta ) so I moved 2 of the Corys to the small tank, all was going well and the fungus seemed to be clearing up. Last night I went up before bed to feed them and ( it's hard to even type what happened) basically my heater had a broke and instead of switching off when reaching the correct temp it did not switch off
, you can imagine what had happened. I am gutted, they were my favourite and most expensive fish in my community tank and I have had them for over a year, I have been unable to find any more the same ( they were sold to me as bandit Corys ). I can't stop thinking about the poor things and what they suffered and I now have 2 lonely Corys left and can't find them any friends 
Anyway, I'm now really worried that this could happen to my big tank, is there anything I can buy that would alert me if the temp got too high in my tank ( something that would beep or flash ), I don't always check my thermometers. I can't risk this happening again.