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In that volume of water, the rate of change should not be a problem. You cannot cool off that much mass very quickly. However, I do not think they will develop well at 15C.
What is the subterranian temperature at 3 feet? If your ground temps have already warmed up, it will partially counteract the low air temperature for a few days. If the air goes to 15C (nights only?) for a day or so, the pond may not get that low. At 17C you would probably be fine.
Keep us posted.
steve hopkins
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