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Man, phosphorus is really tricky. Often, you do not really remove phosphate, you immobilize it by getting it to adsorb to something so it is no longer available as a plant nutrient. However, phosphorus is strongly adsorbed to aerobic organic debris, so there is likely an elevated amount of immobilized phosphorus in the filter backwash. The peat moss trick probably has something to do with the affinity of phosphate for organic matter.
There is a ceramic media which is designed to adsorb phosphate ions called Kent Marine Phosphate Sponge. Its made for marine aquariums and would be too expensive for ponds. I'm not sure how it works, but the ceramic is probably impregnated with aluminum or titanium, perhaps even iron, to increase the rate that the material adsorbs phosphorus. There may be a similar sort of adsorption going on with the bacteria house ceramic media.
Calcium carbonate, such as coral rock or pulverized oyster shell, will immobilize a lot of phosphorus through adsorption, and quickly. This may be the most practical way to lower the soluble orthophosphate. However, the calcium carbonate will also increase your alkalinity (in case you are alkalinity adverse).
Even if you RO the incoming water to remove phosphorus, you still have a significant phosphorus addition via the feed. Feed manufacturers have been working to reduce phosphorus levels, but it's still there.
The active ingredient in Klear is probably aluminum sulfate - its just alum. It can do weird things to the pH. I believe the sulfate is released to the water in exchange for phosphate.
-steveh
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