Wow so sorry for your loss prime,some of the posts recently have been devastating losses,what is it with dats,I lost 3 last week and come on here and OMG it seems like I've had a relatively good week,very strange,don't let it defeat you bud,come back stronger and better.
How long did you not look at the tank for must have been more than a day as the temp would drop slow not over night especially the amount of water you must have had for the amount of fish you lost
It's always a good idea to heat the room with a cheap rad if the rooms warm and well insulated then the temp will drop slow
sorry for your loss it felt bad when i lost 2 and i wanted to get out then so i know how you must be feeling you just gotta pick yourself upand try and carry on.