Hi Marika,
There are some information missing to determine the power requirement for your pump. You will need to know the static lift = elevation from the pond water surface to the tank water surface and you will need to know the pipe/hose friction loss. In addition you will need to know the efficiency of the pump. If you get that information the calculation is pretty straight forward Pm = Head (m) x Flow (m3/s) x density (kg/m3) x g (9,81) – you can read more in the Pumpbook https://pumpfocus.com/pumpbook/centrifugal-pump-losses-and-efficiency/
Hope this help!
3.1.2 Centrifugal pump losses and efficiency