Freshgoby,

The PO4 addition often times stimulates more CO2 uptake.
If not, it has no effect on algae.

If the PO4 is limiting, there's little CO2 demand.
Because the plant can only regulate CO2 uptake by it's most limiting nutrient, the CO2 demand is downregulated.

So if you do not have enough CO2 to begin with, and you are slightly to strongly PO4 limited, adding PO4 will cause a bloom, but it';s not directly related to PO4, it's due to poor CO2.

This why we and most other folks may have very high PO4, say 2-3ppm, dosed 3x or more a week and no algae.

It's not the PO4, it's not having good CO2 to begin with for the cause.

Algae have access to plenty of PO4 either way, but it's when the plants are CO2 stressed that the algae bloom with many species such as GSA, BBA and perhaps a few others.

You can do a simple test and trey this out your self.
Take an otherwise stable tank, add PO4 and keep a close eye on the CO2 when you have PO4 limitation vs none (assume this to be 2-3ppm of inorganic dosing
PO4 per week).

You should see a very notable CO2 decline when you add the PO4.
You should also see some increase in GSA as well, and perhaps BBA.

If you bump the CO2 in response to the PO4 addition, you no long see such algae and you also see a dramatic increase in plant growth and pearling.

This suggest that PO4 cannot be a cause for algae by itself.
It may be correlated in some cases, but it does not express nor imply causation. The question is, if your hypothesis is that additional/excess or high PO4causes algae, then why can I add high levels as I have for 15 years on many tanks and set ups and never have the algae?

This says that the hypothesis is wrong and we must make a new one that better explains our observations.

Such newer hypothesis must be testable as well.
And if they are shown not to be true, we must reject those as well until we have a hypothesis that explains the observations better.

But the new hypothesis I've suggested has not shown to be false.
New planted aquarist with less testing skills might be fooled, but good CO2 control/usage is absoluately critical if you want to test any nutrient in a CO2 enriched planted aquarium.

Adding KNO3/PO4, traces, GH etc will influence the rate of CO2 demand.
Too many aquarist are unaware of this and unable to control and test for it well.

The downstream effects of low CO2 also take a few days to show up, so it's often hard to trace some things to their root causes unless...........you do a test on purpose and wait and see how long it takes for symtoms to occur and know what to look for ahead of time.

Most just try and relate their observations rather than make specific test to show cause or rule out potential agents.

While it may be nice to say PO4 = algae, if that was the case our life would be pretty easy, but this is not the case, the process is more complex and still revolves around optimizing plant growth. Happy plant = happy fish = happy tank = happy owner.

Even in non CO2 planted tanks, the CO2 is stable, low, but stable, then you also do not get algae. PO4 maybe added to 2ppm in such tanks and no adverse effects have been noted. But in both cases, the CO2 is stable and maintained at a non limiting level in one tank and a low limiting but stable level in another.

No such PO4 theory applies nor explains such controlled testing.
Nor does it explain why many Florida lakes that have high levels of both N or P and lots of plants do not have algae, they are literally "gin clear". You can check out several Limnologist's research at UF for more info and background.

For at least a decade now folks in the web hobby have known the PO4 algae hypothesis to be wrong. The researchers have known for about 20+ years.

Regards,
Tom Barr