fin rot? Cotton mouth?

[Jay Hemdal Verified Community Expert]

I'll tag Jay, you may need to raise it slowly. If you have a small wattage heater that may already happen.
@Jay Hemdal

IMO - the OP can raise the temp back up to around 78 F. about as quickly as a regular aquarium heater can do that. This is pretty similar to a rise seen in fish shipped during the winter time (actually, it could be less). Temperature "shock" is not really a thing, as long as it isn't instantaneous. Fisheries biologists usually use 8 degrees F. as the limit for a direct transfer (like when stocking fish).