PhosGuard rapidly removes phosphate and silicate from marine and freshwater aquaria. It is not recommended for phosphate buffered freshwater. PhosGuard is highly porous for high capacity and bead-shaped for optimum water flow. Aluminum Oxide, Soluble Aluminum, and Coral Toxicity In recent years, there has been speculation that aluminum oxide based phosphate removers like PhosGuard release aluminum into the water and subsequently damage corals. The evidence to support this claim was largely anecdotal. So, through a series of controlled experiments in both freshwater and saltwater, we intended to address both the question of aluminum solubility and aluminum toxicity. The results of these experiments show that under reef conditions (pH near 8) there is no detectable soluble aluminum released from alumina. Under conditions of low pH and high dosage levels, soluble aluminum can be released from alumina; at three times the label dosage rate, we detected 0.2 mg/L aluminum at a pH of 5.3. Additionally, aluminum oxide is not easily absorbed into the cell to cause negative reactions. Even at three times the dose of PhosGuard, soft-bodied corals such as Sarcophyton remain unaffected.