Aluminum is an ion that does not get much attention, and has no clear biological use in aquaria. It can, however, have an impact on aquarium organisms if elevated sufficiently over natural levels. Phosguard has been shown to release aluminum to artificial seawater. Further, it appears that the release of aluminum could be the cause of the effects that some folks have seen in aquaria when using aluminum-based phosphate and silicate absorbing materials. However, only a larger study could definitively demonstrate that to be the case. Such biological effects have not been widely reported for the iron-based phosphate removers (e.g., Rowaphos and Salifert’s Phosphate Killer). Consequently, if you are interested in using phosphate-absorbing media, those latter types might be a better choice.