when u replace your carbon filter what do u replace?

it depends on your carbon filter. with some you can replace the carbon, others you need to replace the whole unit. The link you provided is a pre-filter which should be replaced, or cleaned ever so often, whenever it starts to get dusty, clean it, or replace it. Your actual carbon filter should last a couple years depending on what you have and how you use it.
 
Your pre-filter should be changed or washed every few months.
HTG Supply carbon scrubbers are ok for a short while. Replace any scrubber when it stops doing it's job. Go out and smell the vented air, if you smell get a new one. I have heard of some HTG Supply scrubbers only working well for 5 or 6 months. I recommend Phresh filters...
 
Phresh filters all the way, throw the pre-filter in the washing machine every 3 months, the core needs to be replaced every 2 years (more or less depending on usage). Definitely what I'm going with
 
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