I think they need this period of "cooling" as it can reduce the number parasites, kind of like eating White fish, you would never eat them in the summer, yuck! They would taste horrible like "slew water", but what if you catch them in the middle of winter from under the ice, YUM! They are much fresher tasting and free of water borne fea's and other parasites. That's why trout taste so good, they live in cool water. I do love fresh trout fried in butter! When I used to keep them indoors, here many years ago, found that goldfish lived longer outside ..... they always had "health" problems inside, and over crouding was a big issue. Outside they have much more space, sunshine, fresh air, and better diet of fresh greens all year, even under the ice. The outside pond is very little trouble for me, never have to take it apart and clean, large sedges do that for me and good filter too. I just add water as needed, when it is dry and windy, evaportion can be high, although the large sedge plants suck much more water during long dry spells during the summer. Now I only add water once a week.