You'll need to put a thick cover on the pond anyway if you're heating it.
Personally, I would grow the ATF out in a stock tank in the 100-200 gallon range.
Is the pond going to be indoors? If not, what's your plan for heating?
It will be outdoors.
I will be doing the classic pond with a liner and waterfall, but the pond will be insulated under the liner and have a thick cover.
For actual heating of the water, I will have inline heaters with 3-4 watts per gallon.
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