I used to do this every year in Wisconsin.
Because (even for the cold water fish, and water lilies) my 1500 gal pond would become a solid block of ice by Jan.
Fish would go outside in May, stay thru @ mid Oct, and be brought in the basement for the winter
For large fish like koi, I used a 1,000 gal kiddy pool.
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Tropical (and semi-tropical fish) were put in tanks.
But even in summer in WI, the fish kept outside, needed to be quite hardy.
Uruaguayan cichlids worked well, and did northern Mexican species.
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Above Gymnogeophagus in the pond. In WI, even summer air temps could drop water temps down into the low 60s F.
Below Uruguayan Australoheros spawning outside
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Below netting them out for the winter
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