One should never mix different media in a reactor. One has to understand what each media is doing and apply the correct method to get optimum results. Simply adding a media in the belief that a miracle will occur, and that your PO4 is sorted is a dream to say the most.
The reason you media is getting less is because you are flowing/fluidising the media too viciously. It is boiling rather than simmering and pushing through the fine particles. The sponge on the reactor also needs to be able to collect fine particles.
A common issue is that reefers throw in media, and never test if the output is doing what is desired or perceived. It is important to test and control the PO4 (in this case) exiting the reactor/media from method used to apply. One has to slow down, or increase the flow through the reactor so that an optimum of your PO4 requirements are obtained. In other words one needs to test the PO4 exiting the reactor, to fine tune the PO4 consumption from the media and tank. If the PO4 then rises it is time for a media change. When one is able to flow through the maximum amount of water with out PO4 increasing one has reached optimum efficiency.
What i am saying is, when using any chemical filtration, you need to understand what it is doing and be able to monitor the process. Something like adding salt into your aquarium but not knowing what the salinity is, or a setting a heater without a thermometer.
Hope this helps all with PO4 issues and. the use of RowaPhos