what is the best way to keep minnows in a tank cold so they will breed

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I kept my pond cool enough to keep trout alive all summer with no extra energy except the circulation pump. My pond empties down into an underground sub-tank. From the tank, it increases volume, the water flows thru four car radiators plumbed inline and buried about six feet underground, below the pond and waterfall. After the water exits the radiators it is still under pressure and flows out at the top of a five foot high water fall/stream. The little stream is a series of pools and running water. Each pool has smaller diameter rocks and pebbles and finally gravel and acts as the filter. It is full of Crawdads. All this takes up maybe 12 by 9 feet . The water is always clear and while not odorless none of my neighbors have ever complained. The water temp stays a steady 66 degrees all year.
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