Do you mean Poly Filter?
If so I use it, but as a safeguard, not part of my main filtration, as it is too expensive for that. I run this in my sump return section mainly as a tell-tale. When I first set my tank up (it is 15 months old now) it fouled fairly quickly (~2 months) but I am now getting longer life as my nutrient export and filtration is under control. The current piece is 4 months old and looks like it will last another few.
If it starts turning dark brown quickly then all it is doing is filtering nutrients and there are far cheaper ways to do that.
If it turns light brown and gradually gets darker then that's normal and you should just pay attention for other colours. Dark brown again means your nutrient control is not working well enough, other colours mean contaminants of some sort. The instructions explain which colour means what contaminant.
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