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[edit] What is Phosphate?

Phosphates are substances containing phosphorus and oxygen. They are nutrients that are found widely in nature and also in many of the products we put in our aquariums. Phosphate is in tap water, fish food, some salt mixes, additives, and it is produced in the aquarium by all the living creatures.

[edit] Why Test Phosphate?

Phosphates are the primary bio-limiting factors in algae growth. What this basically means is, the more phosphate is in the aquarium, the more algae there is likely to be. Since the reduction of pest algae species (such as Derbesia sp.) is a major goal in the maintenance routine of every aquarist, limiting phosphate levels is high on the desirability scale.

[edit] How To Test Phosphate

Various phosphate test kits are available to the marine aquarist. Some make testing as simple as dropping a couple of tablets into a sample of water, others are more complex multiple-part tests. Whichever one chooses, simply follow the manufacturers instructions and follow good laboratory procedure. Tullock (1997) suggests using a new disposable plastic test tube with each test since phosphate is difficult to remove from plastic or glass surfaces thus making subsequent tests potentially inaccurate.

[edit] Proper Phosphate Level

As with ammonia, nitrite, and nitrate, the ideal phosphate level should be zero. However, since phosphate is constantly being added to and produced in the aquarium, this is a difficult task to achieve. The recommended level for the aquarist therefore should be “unreadable.” That is, if your test kit shows any phosphate at all, you almost certainly have too much.

Note that virtually any phosphate levels can cause undesirable algae growth. An escalating phosphate graph indicates the need for increased water changes using purified water and closer attention to other maintenance practices.

[edit] How To Adjust Phosphate

The best way to limit phosphates in the aquarium is to restrict the amount of phosphates that get into the system to begin with. Since tap water can be a major source of phosphate, I warn aquarists not to use it.

Reverse osmosis (RO) and deionized (DI) water are far better choices. Many hobbyists purify their tap water with both RO and DI prior to using it in their aquariums. This insures a minimum of impurities left in the water.

Other ways to reduce phosphates:

  • Use additives declared "phosphate free" on the label.
  • Avoid overfeeding.
  • Use phosphate sponges or phosphate resins in the filtration system. Two common forms are white aluminum-based beads represented by products such as PhosGuard, PhosSorb and the like, and a reddish granular powder that is a ferric hydroxide compound sold under names such as Rowaphos. Fiber pads such as Poly-Filter and HSH Phosphate Reducer will also extract phosphates from saltwater.

If your phosphate levels are dangerously high, perform a 20% water changes each day using purified water and a phosphate-free salt mix until the levels are reduced.

Carl DelFavero
Reference: Aquarium Keeping and Rescue