It contains a bag of "rainblow blocks" which are essentially sponges. I also have a pre-filter in my waterfall pump. I thought about buying a UV sterilizer. However, because of the expense that has chosen to use beneficial bacteria tablets. I also have water lilies and goldfish varied to maintain the balance of the pond. My pond has a lot of moss, but the water is clear. I would say that both a biological filter and ultraviolet sterilizer can be overstated, but if you can afford both, go!