Water running through the air and bouncing over rocks in a waterfall aerates the water. Sediments get pulled to the bottom of the pond by the pump. The moving water keeps the pond water healthier. Sometimes it's necessary to add a filter to the waterfall to clean out more debris. Make a temporary filter if the pond needs the filter for a short time.
Cut off 6 inches of the foot of the panty hose. Fill it with a good handful of activated charcoal, the kind sold in pet stores for aquarium filters.
Tie the panty hose over the tube that brings the water up to the waterfall with the rubber bands. Make sure the panty hose rests securely on one of the rocks.
Remove the panty hose and charcoal after two days and replace with fresh charcoal. This method works if the filter is needed on a temporary basis.
Cut two holes into the plastic tub. The diameters should be the same as the tubing used to bring the water from the pond pump up the waterfall. The first hole is one inch from the top of the tub. The second hole is 3 inches from the bottom of the tub on the opposite side of the first hole.
Place the tub so it's above where the waterfall starts. Hide it with rocks or plants. Connect the tubing from the pond pump by pushing it through the higher hole in the back of the tub. Connect another piece of tubing from the front lower hose to run to where the waterfall begins. Waterproof around the two opening with waterproof adhesive.
Fill cheesecloth with activated charcoal, the kind that's available at an aquarium shop. There should be enough for a one inch layer on the bottom of the tub. Tie the ends of the cheesecloth to make a bag that traps the charcoal inside. Fill the plastic tub to within 2 inches of the top with lava rocks from the gardening supply shop.
Turn on the pump. The water will fill the tub. As it goes flows over the top of the lava rock through the charcoal it's filtered before it returns to the pond via the waterfall.