I wasn't referring to a fluidized bed reactor. Mine is a packed bed reactor with no movement of the granules. As Paletta mentioned, the problem with these is channeling. The spaces between granules become blocked by calcium phosphate and as a result the majority of the media doesn't get exposed to water flow. I change the media out regularly to prevent a buildup of calcium that blocks flow. I'm sure the media isn't anywhere near exhausted when I change it out but its easier then chiseling it out.

BTW, I was using Phosguard in this application due to the relative cost compared to Phosban. The disadvantage of regular use of a silicate based product like Phosguard is that you are going to end up with a diatom bloom like I now have. I rationalized this as being better then a hair algae bloom, but I'm at the point now that I need to make a change. I think there may be a lower cost Iron based product similar to Phosban coming out on the market soon.