What does your scrubber remove that you need to replace

I've read the thread 'Dripping Potassium & Iron' in the Advanced topics section.
These can be depleted by a scrubber. But not in my system. I measure potassium, no reduction at all. Iron depletion isn't a problem either it seems as my algae growth couldn't be greener.

I do have to add bicarb, specifically as a result of photosynthesis, as I've described in my thread here 'Established Scrubber & Alkalinity levels'.

What elements are removed by a scrubber that should be replaced, especially if no or irregular water changes are performed?

I did read something about iodine & manganese, but it was just a comment in a thread at another forum, so how accurate it is, who knows.