Using only the cotton balls allows for a higher flow rate and makes the tank much easier to move or clean compared to a tank filled with heavy sand.
If you prefer to use sand, you must remove all the cotton balls first and ensure the lateral pipes at the bottom are clean before filling the tank with #20 silica sand.
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By vevor
No, you should not mix sand and cotton balls (filter media balls) in the tank at the same time.
These are two different filtration systems; the cotton balls are designed to replace sand entirely because they are lighter and provide finer filtration.
If you add sand on top of the cotton balls, the weight of the sand will compress the balls, preventing water from flowing through and potentially damaging the pump motor due to backpressure.
By vevor
