Filtering and Carbon and......

I am getting a little confused!
When one has activated carbon in the filter, does it not remove traces
needed by plants?
Does it remove salt, if not why not, as far as I understand, it does
remove metals and surely salt is sodium and chlorine no?
I have recently been impressed by the look of the "Boilife" immersed
wet/dry type filter, except that on the box it says "provides
aeration" How is this possible when the whole thing is submerged? Does
it have some kind of "snorkel" arrangement?
Will it boil off injected CO2? Can a wet-dry be used on a plant tank
without CO2 being boiled off?
What is the best type of filter for plant tanks?
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