Foam is a sign that the filter is not doing its job optimally and there are not enough bacteria to break down the foam. Foam indicates protein, which can be broken down in the filter into ammonia, nitrite and nitrate. These substances will (can) eventually disappear from the water through denitrification, aquatic plants and water changes. In case of foaming, the first solution is to add Marks Gel Bacteria to the pond water for a period of time. This replenishes the bacteria in the filter, which may have (partly) died off during the cold winter months, for example. Read more about Marks Gel Bacteria here: A new product in the webshop: Marks Gel Bacteria for the pond