I know the rule is to NEVER let the ice freeze over. This will be my first winter (-20 C) with a pond and I plan to take every precaution to keep a hole open in the ice, but, what if (god forbid) we are away for the weekend in mid winter and the power goes out. Would my fish be doomed? How long can koi realistically survive once the hole freezes over in a 2,500 gallon pond? Couple minutes, hours, days?