All saltwater aquariums contain phosphate, and you must address this issue for the health maintenance of your aquarium. Phosphate can stress coral and fish, and encourages algae growth. Several methods of phosphate control exist, each with their own pros and cons.
Address phosphate by limiting the source. All living things and fish foods contain phosphate. So, if you avoid overstocking the aquarium or overfeeding, you can limit the amount of phosphate produced.
Making Algae Work For You
Algae absorbs phosphate and you can use this to your advantage. Whenever you scrape and remove undesirable algae, you remove phosphates. You also can plant macroalgae, i.e., seaweeds, which absorb phosphates better than undesirable algae. If you don't like the look of macroalgae, you can keep it in a sump filter or refugium outside the main tank.
More Expensive Equipment
Some specialized pet shops sell a piece of equipment called a phosphate reactor. This consists of a chamber where aquarium water reacts with chemical media to remove phosphate. Phosphate reactors work consistently, but are costly and tricky to set up. Protein skimmers, a more common piece of aquarium equipment, also reduce phosphate.
Kalkwasser: One Stone, Several Birds
Reef aquariums often receive calcium hydroxide supplements. Calcium hydroxide offers many benefits in aquariums, including buffering the pH and adding calcium for corals. As an added bonus, it binds to phosphate and renders it chemically inert. However, it takes practice to dose it correctly.