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I think based on your system, and your past comments with cyano and others, that the Chateo is really NO3 limited..........removing more PO4 is not going resolve it either.
Just leave it alone, feed the tank more.
This is a well supported in aquatic systems amd seemed really missed by most on this forum and elsewhere when it comes to noxious micro algae vs the much large macro algae:
The smaller the the organism, the easier it can grow and compete at lower concentrations of nutrients/resources.
So BGA can compete and live at much lower ranges than Chaeto.
They are not even in the same niche ecologically for that matter.
Rather than obsessing with less PO4 and less NO3, and believing less or none is "best", think about it.
Both need N and P to grow.
Yet one can survive beyond the range of any testing equipment you own(unless you do aquatic pelagic algal research), the other needs a fair amount more, about 10-100X more.
The other issue that befuddles the marine aquarist and macro algae/noxious pest algae for that matter: nutrients are rapidly, and lot more so rapidly utilized and recycled at a MUCH faster rate when the tank is lean to start with.
So if there is not much available, then the nutrients are going to be used much much faster, so as fast as it is excreted as waste or dosed to the tank, it will used and the test kit will never measure a thing in the water column to start with, thus bioavailability and what is the residual in the water column are not even correlated.
As you increase the nutrients, say adding KNO3 or KH2PO4 to see, then you'll note a residual that you can measure. There's enough there to see the cycling and uptake over a unit of time.
If you are talking 10ppb, then things happen much faster.
There is also the issue or Organically bound forms of N and P and how fast they are mineralized. Few test kits made for hobbyists differentiate between these forms and bioavailability etc.
This is an old old issue for researchers however.
Still, it applies here the same.
You can also see a push pull effect on PO4 by adding NO3 to such systems that are NO3 limited. Then the PO4 levels plummet.
It's very difficult to see if the ratios of PO4 or NO3 are limiting in such super lean systems. Noxious algae like to bloom/germinate during such transitions and use these changes as germination signals during seasonal and other events in natural system.
Means it's a good time to grow and complete a life cycle.
Chaeto will leach out nutrients in lean systems also as it's not growing well, so often the BGa will grow near/on it. Organic material in the sediment also allows this to happen as well as providing a good surface substrate for the BGa to grow as well.
With good growth of the macros, you will have little noxious algae.
They(noxious algae) can "tell" that someone else is there and growin well. Keep it stable and do not obsess with zero ppms for NO3 and PO4, low is fine, but never absent.
Clean it out every so often and remove excess growth and organic matter, that will go a long long way to helping the macros serve you much better.
Regards,
Tom Barr
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