I only use them if i am trying to take something out of the water like excess meds or if i have put in some new wood which is leaching excessive tannins. The problem with them is that they get full and then (carbon especially) will dump it all back into the water in one hit. Thats is why i use it for a week then toss it.
If you dont like the tannins then take your wood out and boil it for a hour or so, this will get the tannins out fast!
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