I have my 100 gallon koi tub going with pump and filter on the back porch. We've had a cool spring so far. The water is pretty chilly! the koi just huddle around the base of the pump. I'm assuming it must generate a little heat.

Can I, should I, add a small heater or warmer water, if I want them to move around, generate hunger, eat and grow? Any reason for me not to control the regulation of their environment?

Any opinions?