The majority of pores or holes in feather rock typically are closed end. The hole does not go completely through the rock. It should also be noted that it is a natural product that can vary greatly in the pore length and depth. It's been a long running theory (but never proven) that the deep pores fill up (clog) and this is what provides the denitrification ability. Seems to me it should be easy to prove by measuring water in and out, but no evidence yet. Personally I don't know and also don't think it's a general statement of the entire media clogging.
If this process is true there would be lab evidence and large scale commercial adoption by now.
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