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Goodfish... as often as they need it. If they stop working or flow is severely restricted... then pull them out and clean. Many people will let their PH's gather coralline so they blend in with the rest of the tank.
remember, all you really need to clean is the inlet, impeller, impeller cavity, and the outlet. you can leave the body encrusted in coraline. only the working bits and the water flow path need to be cleaned.