Question
I live in toronto and I have a fish pond in my garden,, the dept of the pont is 4 feet and width of the pond is 8 by 10 feet,,question is that the tempoerature is droping below -3 and lower,,at what temperature do I need my water heater ( the fish in the pond is Koi fish,,22 in nymber)

Answer
Hi Jerry,
Thanx for your question.  I would put the water heater in now.  All you're going to do with the water heater is keep a small part of the pond's surface from freezing over.  If the pond were to freeze over the fish would suffocate from the gasses that are constantly passing through the water from decaying bio material.  The fish will be fine even at colder temperatures.  They have a kind of antifreeze in their system.  They may slow down and not move around much but they'll be all right as long as your waterheater keeps  a small area open so gasses can escape.  I hope this helps.
Tom