Pressurised filter for adding to your existing biological filtration. With the duel valve/by pass system it means that your whole of your flow can be put through the system, but only the amount you want to go through the fluid bed, the rest is passed via the by pass valve back to the pond where it joins the water treated by the filter, this means no unwanted restrictions or no second pump! Great idea for overstocked ponds , or people feeding alot of food.