Normally the best time to perform a pond clean-out is the early spring, before your water garden completely awakens from its winter dormancy.
If a clean-out is performed when the water is warmer, after bacteria colonies form, the balance of the ecosystem will again be thrown off and your pond will go through another “green phase” before the bacteria colonies re-establishes themselves again.
But this does depend on how long ago the pond was cleaned out. And The condition of the pond and the health of the fish. If the pond is in a bad condition or the fish are not healthy the pond should be done straight away because the waste is giving of methane gas it’s stripping the oxygen out of the water.so does not matter what time a year it’s done.