How often you need to clear your mechanical filter box will depend on how many koi you have in your pond. Rule of thumb – once a week should do it. Watch your filter outflow. If there is a reduction in water flow or pressure, the filter needs cleaning. Pond filters have mechanical and biological filter systems. Mechanical filter clears out solid waste, and biological filters use beneficial bacteria to change toxic waste into less harmful nitrates. However, if too much sludge is allowed to collect in the mechanical filter, it will slow the flow and start to break down, creating phosphates.