From what i hv read, more than half of the GAC for aquarium use in the market either contain phospate or will leech phosphate. That's why it is always recommended to rinse the GAC in freshwater or RO/DI water b4 use.
webbies to stake my claim.
An Overview of Activated Carbon in the Marine Tank By James R. Layton
unedited article in Aquarium USA 1994
Abstract from Seachem MatrixCarbon:
A truly unique activated carbon, bead shaped for optimal hydrodynamics. This provides maximum water flow and contact with a high density of adsorptive sites without the compacting common to granular activated carbon. MatrixCarbon has a very low ash content as indicated by its minimal impact on pH. Even when added to distilled water, it does not raise pH above 7.0. While all carbons contain phosphate, regardless of false claims to the contrary, MatrixCarbon has the lowest leachable phosphate content of all major carbon brands tested. It out-performs other best grade carbons by at least two-fold when compared for total capacity to remove aquarium organic matter, rate of adsorbtion, and duration of use. 250 mL will easily treat 100 gallons for several months. It is very clean and nearly dust free. The 100 mL bag size will fit any filter on the market and treats over 40 gallons. Marine or freshwater use.