Need to warm a pond how to calculate it ?

Hello everyone
I need to warm water for fish ponds.
The fish pond is located in a building that is not isolated.
My problem is how to calculate the system capacity when the cold load changes during the day.
Can anyone tell me how to calculate?

Thanks.
Robert

Need pond dimensions, room temperature, pond temperature. Is pond insulated? surface area? Is pond in the ground or on top of a floor?

I need to heat a pool ,can you suggest me how to calculate the needed heat capacity or heating input power, many thanks!!
1. 75 cbm(area about 50m2)
2.The pool is outdoor.
3. Winter ambient temp is 10 degrees and summer is 32 degrees. The pool will need to be used 8 hrs a day - only in the winter
4.The temp to be 20 - 23 degrees in the pool