I have seen reports of reactor media turning to mush when ran at too low of a PH, but that has not been my experience. I have a saturation style reactor, A.E. ACR, that runs a PH < ~6.0. I use old school TLF in it. For a secondary chamber, I use a Geo 618, with no CO2 hookup, just the circulation pump running, and it's PH hovers between 6.1 and 6.13. In the Geo I have 90% TLF with a thin layer of TLF ReMag, ARM Coarse and SeaChem Reactor. The flowrate through the CaRx is a very low 1.3 ml/min.
I decided to add the extra media types in the secondary chamber just to try to see how they melted compared to each other, also they each claim to be particularly good for different elements (ARM, extra Strontium, Seachem, extra Potassium, ReMag, obviously magnesium).
Just from visual inspection of the layers, the ReMag is definitely, melting the fastest. It's layer is merging into the ARM. The ARM appears to be next for the amount melted, followed by the Seachem. The ARM and Seachem have become noticeably smoother, sharp corners and edges melted away. The TLF Reborn in the CaRx and the secondary hasn't changed in appearance, or volume at all that I can see.
My conclusion is that any issues with media breaking down, turning to mush, has more to do with the waterflow through the media, than the PH that the media is ran at. Both my CaRx and secondary are up flow, and have a good amount of circulation through the media.
Dennis