Thanks mpmister! I'll check the levels. I use a tetra conditioner, but will definitely check out prime, and have to use a heater to keep it within that temp range (my dad keeps the house very cool). What freaked me out the most was that no significant change had been made to the tank, and there have been periods of time where a filter has had similar problems and nothing like this has happened. I guess I've just been lucky up till this point and I may even buy a backup filter like you suggested. Thanks again!