Large or small, greenhouses are a good choice for culture of warmer climate fish and plants. With the right combinations, the greenhouse can be in use year-round, and you can even develop a closed loop system within a greenhouse.

Many people set up very artificial systems within a greenhouse, and that is fine, if that is what you can build, but it is also possible to set up something much more natural and self-sustaining if you wished.

Greenhouses can be effectively used without heat, or with passive heat, and if you have the resources and something really worth doing, you can put in some kind of fueled heat - but I would not do so unless I knew that it would more than pay for itself. Energy is just too expensive to throw around, when a little creativity can avoid it.

For a Greenhouse system, TEMPERATURE is the big issue.

In the summer the greenhouse WILL OVERHEAT if it does not have both SHADE, and COOLING. Cooling is generally done by fans, and the shade should be OUTSIDE, not INSIDE, which means, a shad cover thrown over and tied down is more effective than a shade cover fastened INSIDE the greenhouse. Shade is more effective if applied BEFORE the heat magnifies.

In the winter, even a well insulated greenhouse can freeze overnight. It is good at GENERATING heat in the daytime, but it is not good at RETAINING heat overnight. So winter extremes may be BOTH too cold, and too hot, for some kinds of aquatic life.

Fish and plants tend to be either WARM CLIMATE, or COOL CLIMATE, and they are RARELY a thing that can handle extremes of temperatures.

The greenhouse environment is best when maintained for the winter, and removed for the summer, and that's a royal pain. In the far north, a greenhouse can operate all year for temperate livestock.

Water mass is a good passive heat reservoir, and it can serve as a heat sink. All this means is that for temperature regulation, large amounts of water work better than small ones.