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I like your analysis. A lot, actually. :)

The consensus answer in this hobby, or at least in this forum, would be:

1) No distinction is ever made between nutrient forms. All phosphates are treated the same, and there is no reason to distinguish between them.
2) At least 20ppm nitrate, 2ppm phosphate, and 20ppm potassium are required to avoid deficiency in all plants. As levels drop below that, progressively more plants encounter difficulty uptaking nutrients at sufficient rates.
3) However, tests are too inaccurate to be of any use in determining whether you actually have these minimum levels. Even if they were accurate enough, it's a waste of time to test. Instead, simply add an excess of all nutrients, such that there is no possibility of any nutrient dropping below the minimum level.
4) The excess nutrients do not cause algae.
5) Although eventually, nutrients will build to toxic levels, and large excesses (as well as other accumulated wastes) are periodically removed by performing a large regular water change, typically 50% weekly.

As far as advice goes, the above is practical and works almost universally. I use it in my own aquariums, and you should too.

However, if you want to experiment, look a little deeper, and try to draw a few extra logical conclusions, you will find the success of the same advice will hinder you, because it is accepted as gospel. The common responses to anything that challenges it as incorrect or incomplete are dismissive (why bother when what we have works), nonsensical, and sometimes even hostile.

Here's two replies I see a lot. Given your analysis, I'm sure you can spot the logical errors within:

"When plants are healthy and growing fast, they out-compete algae for nutrients." This comes from people who dose nutrients in excess, such that none are ever limited.

"I've dosed huge excesses of potassium phosphate, far beyond anything an aquarist would encounter. No algae resulted. Therefore, excess phosphorus does not and will never cause algae." This one comes from the same aquatic biologist widely credited with developing the advice I listed above.

Since you're new here, I figured you should know what you're up against in attempting to start a discussion of this nature. :) I typically avoid it myself, but like I said, I liked your analysis.

That said, I agree with you at least in theory that nutrient form matters. I have repeatedly witnessed different forms of phosphorus having particular effects not accounted for by present theory.

However, I don't know if the phosphate form in your water supply is in fact unusable by plants. The person here most qualified to give you a scientific answer is the same biologist I mentioned. I have attempted to open discussions regarding different phosphate forms with him and others in the past, and received only answers that don't address the question, or contain further logical errors that make the response suspect at best.

I do think that you cannot rule out the possibility that in your aquarium, phosphate becomes so sparse at 0.5ppm that absorption might simply slow to the point where it effectively goes no lower.
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