EDIT
I just saw your comment:
It is less of a pond and more of a rectangular hole in the ground. I am using it as a temporary tank until I finish my full pond
It is going to be much less stress and a lot more economical to finish first your actual pond, and use it for some time (does it need cycling, like aquariums? I do not know). And when things are finished, "invite" the fish to the pond. Do not start with buying the fish, and then continue by wondering what to do with them. Just a friendly idea.
1 meter below ground surface is definitely NOT underground. Just barely in the ground. You did not mention where in the world you want to grow this business, but be aware that in some areas, the water can get a layer of several cm (or more) of ice on top - if the tank is left open.
I will continue the answer with regard to temperature control, not about the fish. You will adjust the parameters according to your needs.
Your problem depends not only on what the fish need, but also on the tempratureS of the environment. Please notice the plural. Your "tank" touches the ground as well as the air (at least, in the picture you provided). You can have a cold ground and hot air during a hot spring day, and you can have warm ground and cold air during a weather anomaly in the summer.
So in order to maintain the temperature, you need insulation, before applying heating. The easiest insulation against the ground in polystyrene. There is a good variety of materials end technologies to choose from. please remember to cover both the bottom and the walls. You can create the walls above the ground (see below) at this step too - if you plan carefully.
The insulation against air requires a roof and walls - and these will also prevent dirt and garbage to fill your tank. During warm weather, the walls should remain open for fresh air. During the cold weather, close everything.
The heater on the bottom of the tank should be controlled by one or more sensors, to keep the temperature in the desired range, and the cost of the heating low. be careful that some heater can get very hot themselves, and therefore hurt the fish - I o not know what kind of technology you intend to use.
Additional notes:
- you might need to create some water movement, for the comfort of the fish;
- most likely, you will need a filter - which will create a water flow anyway - and it should move the entire water, not only a small volume around the filter;
- feeding should not let a lot of heat being transferred - you need to open the enclosure somehow. You might add a special feeding "tube", and you open only that tube during feeding.
- if you use a filter (and you should), then you only need the heater at the output of the filter, not on the entire bottom of the tank.
- of course that sooner or later, you will need access inside yourself. Plan the setup accordingly.
- most likely, light is an important factor. You might not need all the complicated technology used for house windows, but the setup with a sandwich of layers of glass and air would be a good start. depending on your particular details, use glass only where it makes sense - most likely, towards south). I hope you dug the hole taking in consideration the needs for light too :)
