I can only tell you what I would do - if it lasts for more than an hour, wrap blankets around the tank and start doing small water changes with warmer water every hour - not an 'educated' opinion, just common sense I suppose.
it takes a long time for the water in the tank to change and its a gradual change unless its a very long time it shouldnt be bad, but also i live in florida and my tank is running at 84 with no heater now so i cant tell for up north
