I don't think there is any need for carbon, especially in a planted tank, however I don't believe it will interfere with ferts. The only time you might need carbon is if you are removing meds, unwanted odors, or possibly tannins in your tank. Most of the rest of the time carbon is not really neccessary and in fact, loses its effectiveness after a few weeks. I am assuming your Whisper EX filter(s) uses "disposable" cartridges with carbon already in them?
I run Aquaclears on my tanks as I love the variety of filter materials available for them...plus the Media lasts a long time and rarely needs replaced. My son recently got his own tank. It was a Craigslist buy that came with a Top Fin Whisper HOB
Filter that was designed to be used with filter cartridges. It has plenty of space inside it though, so instead of purchasing more cartridges for it, I took extra foam and BioMax media for my Aquaclears and put that inside the filter instead. Idk if that's an option with your filters or not, but I thought I'd share.