Funky Fish - I believe you need to do more research with regards to carbon use. Many Granular Activated Carbon contain phosphates left over from the manufacturing process, and can add phosphates back into to the water column.
You will find that warning generally right on the manufactures page. Here is an example of one on j&l.
http://www.jlaquatics.com/phpstore/s...t_ID=sc-mc2000
Now while this is not the Carbon i use. I personally use Kent, which is suppose to be Non-Leaching. I don't like to leave it up to manufacture to always tell truth. All carbon has phosphates in it so i'm adding a stage of "piece of mind"