Thanks, Patrick! It's a DIY fluidized bed filter using K1 media. The idea is that the media in the bottle is constantly moving which promotes the growth of new and more efficient benefitial bacteria. It sits on top of a sponge filter which provides mechanical filtration. Plus it looks kind of neat 🙂 and you can take out some of the cycled media and use it to seed new tanks. Here's a video of Cory making one, just in a slightly different way: