MBBR water treatment process

We found this waste cleaning system at Maryport Aquarium. There is a biofilm on the plastic shapes and the microbes can process substances in the water. The plastic is less dense that the water so it should float. There must be something constantly pushing the plastic shapes downwards. This film shows little fans doing just that in two of the tanks https://www.youtube.com/watch?v=BduxDqwmBHg The upwards resultant force on a plastic piece would be upthrust - weight. By Archimedes' Principle, upthrust = weight of water displaced. Density of water = 1000 kg per cubic metre. Plastic is 950. So upwards resultant force = (volume of plastic x 1000 x g) - volume of plastic x 950 x g) = Volume of plastic x 50 x g. The closer the densities, the lower the resultant force and the smaller the acceleration with which the plastic rises to the surface.