Minor Tank Disaster: Heater Blew Up

[kien]

could you post a picture of this black stuff ? That would help.

74 degrees isn't that cold. I suspect the temperature dropped gradually so it probably didn't shock your fish. Last year my tank lost power for 24 hours and my temperature dropped to the mid 60s and I only lost 2 fish (but that was probably more due to lack of oxygenation).