Found this reefkeeping website: http://reefkeeping.com/issues/2006-09/rhf/">http://reefkeeping.com/issues/2006-09/rhf/</a>
"Phosphate Export by Organisms: Macroalgae
Growing and harvesting macroalgae can be a very effective way to reduce phosphate levels (along with other nutrients) in reef aquaria. In my reef system, where I have large, lit refugia to grow the macroalgae Caulerpa racemosa and Chaetomorpha sp., these algae are clearly the largest phosphate export mechanisms. Aquaria with large amounts of thriving macroalgae can avoid microalgae problems or excessive phosphate levels that might inhibit coral calcification. Whether the reduction in phosphate is the cause of the microalgae reduction is not obvious; other nutrients can also become limiting. But to reef aquarists with a severe microalgae problem, the exact mechanism may make no difference. If rapidly growing macroalgae absorb enough phosphorus to keep the orthophosphate concentrations in the water column acceptably low, and at the same time keep microalgae under control, most reefkeepers will be satisfied.
For those interested in knowing how much phosphorus is being exported by macroalgae, this free PDF article in the journal Marine Biology has some important information. It gives the phosphorus and nitrogen content for nine different species of macroalgae, including many that reefkeepers typically maintain. For example, Caulerpa racemosa collected off Hawaii contains about 0.08 % phosphorus by dry weight and 5.6% nitrogen. Harvesting 10 grams (dry weight) of this macroalgae from an aquarium would be the equivalent of removing 24 mg of phosphate from the water column. That amount is the equivalent of reducing the phosphate concentration from 0.2 ppm to 0.1 ppm in a 67-gallon aquarium. All of the other species tested gave similar results (plus or minus a factor of two). Interestingly, using the same paper's nitrogen data, this would also be equivalent to reducing the nitrate content by 2.5 grams, or 10 ppm in that same 67-gallon aquarium."
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Summary of Phosphate Reduction Methods
My suggestion is for aquarists to target a phosphate concentration of 0.02 ppm phosphate, or less. Here is a list of ways that many aquarists export phosphorus and maintain appropriate phosphate levels. They are listed in order of my preference for addressing these issues in my own system:
1. One big winner is macroalgae growth. Not only does it do a good job of reducing phosphate levels, but it reduces other nutrients (e.g., nitrogen compounds) as well. It is also inexpensive and may benefit the aquarium in other ways, such as being a haven for the growth of small life forms that help feed and diversify the aquarium. It is also fun to watch. I'd also include in this category the growth of any organism that you routinely harvest, whether corals (e.g., Xenia sp.) or other photosynthetic organisms.
2. Skimming is another big winner, in my opinion. Not only does it export organic forms of phosphate, reducing the potential for them to break down into inorganic phosphate, but it reduces other nutrients and increases gas exchange. Gas exchange is an issue that many aquarists don't ordinarily recognize, but it is the primary driver of reef aquarium pH problems.
3. The use of limewater, and possibly other high pH alkalinity supplements, is also a good choice. It can be very inexpensive, and it solves two other big issues for reefkeepers: maintaining calcium and alkalinity. Simply keeping the pH high in a reef aquarium (8.4) may help prevent phosphate that binds to rock and sand from re-entering the water column. Allowing the pH to drop into the 7s, especially if it drops low enough to dissolve some of the aragonite, may serve to deliver phosphate to the water column. In such systems (typically those with carbon dioxide reactors), raising the pH may help.
4. Commercial phosphate binding agents clearly are effective. They can be expensive and may have other drawbacks, but can drive inorganic phosphate to very low levels, if that is a goal.
5. Driving bacterial growth is another option. Not only does it do a good job of reducing phosphate levels, it reduces other nutrients as well (e.g., nitrogen compounds). It is also very inexpensive and may benefit the aquarium in other ways, such as providing a food source for certain organisms. Its drawbacks are that it makes it difficult not to drive the nutrient levels too low, and the fact that it consumes oxygen as the bacteria use the added organics as a carbon source.'
