Fe+2 is ferrous iron is the soluble form
Fe+3 is ferric iron and is inthe precipitated form.
If you do a pre-filtration and the filter media is fine enough you will remove most of the of the Fe+3.
If you want to remove ferrous iron you have to oxydize it before removing it because it is in solution and that can be done with aeration or with maganese permanganate or maganese dioxyde catalyst.
I don't think that the pre-filtration will affect the ferrous iron oxydation.
Thanks for the answer! Since asking, I read a bit more and came across something that mentioned Fe+3 could act as a catalyst for Fe+2 oxidation.
This was a bit over me, but I got that impression from page 259 here: http://www.jbc.org/content/90/1/251.full.pdf
I read a bit of the paper. It is a lab experiment and they used a lot of chemicals not use in water treatment. What is your problem exactly you have a water with iron? Can you tell us more?