I have two 2215 filter and I remember reading somewhere that it's recommended that you replace 50% or so of the bio media (ehfisubstrat/pro) every year or every so often.
I was wondering what's the reason behind this. Is it only because the pores of the media get clogged up or is there another reason?
Personally, I think this is quite excessive/expensive. All I've done with the bio media is give it a good swish in old tank water. It knocks loose a lot of the gunk that builds on it and it comes out cleaner than when I put it in. I might just buy a gallon or two of plastic bio balls to replace them.