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Phosphate (PO43-) has always been labeled as the “bad guy” in reef-keeping. The truth is, phosphate is an essential element used by all marine life. It’s essential for fish, corals, coralline algae, clams and even the microbes that keep the reef tank clean, clear and healthy. Phosphate is used by zooxanthellae algae inside LPS and SPS corals. Without phosphate the corals would die. But phosphate is also used by marine algae. While algae play a vital role in the marine food chain, we call them a nuisance when they show up in our reef aquariums. But don’t blame the algae for thriving. They’re just responding to the conditions we give them in our tank.
Remember, zooxanthellae algae like the same things as most other marine algae. Give them light and nutrients and they’ll grow. But who wants an algae-covered reef? In nature we see that the reef rock is free of algae but covered with thriving corals. How does this happen? It’s a combination of low nutrients and algae grazers that continually peck at the rock, keeping it clean. Is this possible in a reef aquarium? Yes! Here’s how.
We don’t want to eliminate all phosphate from the water. We just want to “balance it out” so algae can’t take over the reef. This is done by several methods. Start by limiting food input. Fish and invert foods contain phosphate, because it’s part of a balanced diet. But too much just pollutes the tank. Protein skimming removes phosphate-rich organics before they break down into inorganic phosphates, which algae thrive on. And the last step is to physically remove inorganic phosphate from the water. What’s the best way to remove phosphate from the aquarium water? Granular ferric oxide or GFO for short. GFO is a reddish-brown granular material, somewhat like activated carbon in size. It’s placed in a mesh bag and dropped in the sump or power filter. GFO can also be used in a filter sock, media contactor or reactor unit. As water passes through the bed of GFO, inorganic phosphate binds to the iron, removing it from the water. Lowering the phosphate helps to inhibit algae growth in the reef aquarium just enough to keep it under control by grazers. So how much GFO should you use? Here’s what you need to know.
Granular ferric oxide removes inorganic phosphate, some alkalinity and trace elements. It doesn’t just go for phosphate, so calculating exactly how much phosphate a certain amount of GFO will reduce is very difficult to accurately predict. Also know that phosphate is continually being added to the water through fish and invert waste and invert foods. If your tap water contains phosphate, every top-off and water change contributes to phosphate loading. Our best recommendation is to start with our GFO Media Calculator. Just put in your aquarium size and the calculator does the rest. We recommend monitoring pH and alkalinity (carbonate hardness) when using GFO products. It’s also helpful to test for phosphate while using GFO so you can track the results. Fine tune the amount of GFO you’re using and how often you change it out, based on testing.