Bummer. :( I wouldn't beat myself up over it, but you should take the lesson for everything it's worth: it's always risky to put a PC or any other cheap complex device in a critical real-time control role. Even when "real time" implies a lag time of an hour or more.

A good cautionary tale in fail-safe design. You're now a better aquarist and a better engineer. Go forth and sin no more. :-P