Sold! Thanks very much.
Steve, I didn't use it long enough to form a really solid understanding, but, it seemed to work OK. I think it is at the very least comparable to a downflow. If anything, perhaps a little more efficient on account that part of the media bed gets fluidized (and prone to less clogging over time). That said, I don't know if there are any long-term significant differences. One nice thing is you don't need a recirc line for the CO2 at the top, it never gets a chance to accumulate.