All of these answers are relative to many thing, like,
How heavy your bio load is.
How often and how much are your water changes?
How much live rock you have.
Are you sure you RO waters TDS is 0?
IMO, how long your light cycle is
My stuff gets so dirty in my 28g that I scrub the skimmer and media basket 1x a month. I replace my chemi pure elite every 3 months even though it's supposed to last up to 6 months. I would see what they say and replace it no later than 3/4 of the instructed time.
I would always use carbon or chemi pure elite because there are so many issues that can dirty your water that it's impossible to know unless you test constantly.
Take it from me, I've had mini crashes because I thought I could do without...
It seems like the minute you get confident that everything is going really well, poof, something happens!
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