Those wagon wheels are called K3 bio media. They help the nitrification process of the ammonia, and a vital part of the system because it's Aeroponics.
Bacterial matter builds on all the surface area though out the system. Like in any Aquarium, this bacterial matter changes Ammonia to Nitrites, and after O2 is introduced, into Nitrates; and what plants use to grow. The more of this bacteria, the higher the load of fish can be used, and the more Nitrates for the plants.
K3 media has a ton of surface area. It's also neutrally buoyant, after the bacterial matter collects on it. It moves to make the contact with the most water.
Normally Aquaponics is designed with Ebb&Flow flood tables, with hydroton and rocks. In those systems, any media in the grow beds act as a filter or more "surface area". Since my system is Aeroponics, with no grow beds, this was one of my only options.
In all my videos I try to show the path of the water flow. I hope it's not too confusing. The filters I use now are maxed out for what they can handle. In a future video, you'll probably see it swapped out for 2-3 times the media, in an IBC tote.