My worst nightmare regarding fish is slightly different and is still something that worries me...
A power cut to the house, or just the Blagdon electric entension to the garage tripping (leading to suffication before ammonia or nitrite issues)
The current cold snap being too much for my Rena 300W and Juwel 250W to maintain 22C in the uninsulated garage, leading to a gradual freeze (I feel more concerned for "Simona"'s Discus and "Mattlee"'s Oscat tanks that need warmer temps and I know simona had heater isues that resulted in ~23C water for his Discus in Wirral!
)
I really must get a ~2000vA Uninteruptable Power Supply sorted out from Ebay, but I don't have ~£150 of "disposable cash" (wish I had now hinted at fish related things for Xmas, there are several things I would like and simply cannot afford right now).