I need to maintain 400 liters of salt water at 6°C in a closed system. Outside air temperature at peak cooling periods averages between 18 and 22°C, but can be as warm as 30°C. The tank is 12mm molded plexiglass. Commercial cooling units rated for sea water available here range in power output from 350 to 4000 watts. How much power do I need? (Circulation pump achieves a throughput of approx 350L/Hour, if that is indeed relevant).

Thanks for your help.