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Discussion Starter · #1 ·
What level of phosphate is typical in a NPT?

It seems that the potting mix I am using has a lot of phosphate in it. The level in both the tanks I measured is around 3ppm. One of the tanks I just set up yesterday, the other has been running for months and had a lot of water changes in the first couple of months.

I believe this level is higher than advisable for a planted tank. But then fish and plants both seem fine in the established tank, and while I have some hair algae growing it's not taking over the tank or anything.

Does anyone know what level of phosphate is usual or advisable in a NPT?
 

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Phosphate in the soil is good. Plants like to get phosphate from the soil rather than the water (my book, p. 106). Also, phosphate (unlike nitrate) generally stays locked chemically in the soil and its presence (unlike nitrate and ammonia) doesn't cause fish toxicity problems.

When I set up my 45 gal in 2001, I mixed a little phosphate fertilizer (2 tablespoons of triple phosphate fertilizer) to the potting soil. Yes, this was probably too much and I had initial trouble with algae, but the plants grew like crazy, and they're still doing well in this tank.

When I reset up my 50 gal this week, I'll be adding phosphate fertilizer again to the potting soil.
 

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Discussion Starter · #4 ·
The phosphate in my soil seems to get into the water easily, though maybe it's only a small amount compared to the total amount in there.

I've got about 3ppm or more in both my NPTs that I measured. One of them has had a lot of water changes, though not recently, and while I was doing lots of water changes the phosphate went much lower. I didn't realise until the other day that it had crept up again - I guess it is still coming out of the soil in relatively large amounts.
 

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The phosphate in my soil seems to get into the water easily. I guess it is still coming out of the soil in relatively large amounts.
Phosphate release from a freshly submerged soil should slow with time as the soil "settles down" in the coming months. After that, fishfood will start adding phosphate to the water. Much of this phosphate will precipitate to form various calcium phosphates and iron phosphates. These precipitates will collect in the soil (as plant nutrients!) and biofilter.

Bottom line: I wouldn't worry about phosphate in the water (some of my tanks often have more than 3 ppm phosphates). In aquariums (unlike natural waters), the limiting nutrient for algae is iron, not phosphate (see my book, pp 160 and 167).
 
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