Thank you for using our fertilizers. Because of the way that our products are formulated, the plants are able to utilize the nutrients and uptake them very quickly. Therefore, carbon should really not be able to pull them out of the water completely. With that said, I might suggest that instead of using carbon, you go with Purigen. Purigen does an excellent job of removing organics, and because it has a specific affinity for nitrogenous waste, it will have a very minimal impact on any of the fertilizers that you are dosing. If you would like to read about Purigen, please see this link: http://www.seachem.com/Products/prod...s/Purigen.html
As far as water changes are concerned, I would recommend more like 20-25%. Please let us know if you have further questions.