Yes, as I have used the filter guys gfo.. You need to have it turned down a lot from where you have it. The surface really shouldn't even "bubble" that much. I used to try to get "all" of it in the chamber to tumble or fluidize, but that's to much and you will end up with dust..
I always found it confusing as they call it tumbling. But it's really not. And the flow is more like carbon, as your just using the reactor to get maximum contact(but it remains still because the flow is still low) , than actual tumbling like with pellets pellets.
The thing I do now is test the effluent from the reactor. Once it hits zero, I don't even concern myself with the tumble rate.. As for me, way to confusing without some kind of flow meter to measure.. I then simply measure that effluent every couple days to make sure it stays the same. Once it goes up time to replace..
But the brs hc is my next step.. I want to use less in the same space.. And it is a harder consistency. But, if it's grinding (any gfo), then it's being pushed to hard and fast..