yah even then I'd take those readings with a grain of salt. I have recently done a tremendous amount of research in to phosphates for some soil analysis stuff I need to do for my masters. It's pretty epic, not at all like nitrate which is a straightforward, water soluble compound. Phosphate can either be organic or inorganic, and within that there's like 3 dozen different forms it can take depending on what it's bound to. On top of that there's an ongoing and dynamic process of conversion between many of the forms on a pretty constant basis, and many of the forms it can take are available to plants, just in varying degrees.
The process for testing ALL of the labile (ie, plant available) phosphorous in a sample of soil takes an entire week of running sequential fractionations in a lab with dozens of different acids, reagents, resins, etc, so it's a safe bet that the single shot, one or two reagent hobby grade test kits that we use only give the tiniest picture of what's actually going on from a phosphate point of view.
If you've got out of control algae growth, and your nitrate tests are showing zero, chances are good you've got phosphates that your test kits simply aren't sophisticated enough to pick up, or it's being consumed as soon as it's produced.
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