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PO4 Deficiency & CO2 Swing

I have made some interesting observations about my tank. There was a time
when I was getting very large pH swings of about 0.6. I'm using yeast CO2.
Now my CO2 concentration is very stable throughout the day, and pretty high
too, around 30 ppm. I haven't made any changes to my injection method. In
fact, my current yeast mix is getting pretty old and the output is pretty
low by now. I noticed this stability in pH a couple of weeks ago. In
addition, all my plants are growing very slowly. My Ambulia, which used to
double in size every few days, has almost completely stopped growing. My
plants weren't pearling a lot either.
I suspected that it might be a deficiency in some other nutrient that is
preventing the plants from taking up a lot of CO2. Since I already
supplement the tank with KNO3, KCl, and MgSO4, one nutrient that might be
lacking is PO4. If that is true, it is a big achievement for me because I
don't believe I've had a limiting amount of PO4 for more than short periods
of time in the past. So what I've done is added a pinch of a phosphate-based
buffer I've had to the tank. I've done it 3 times over a couple of weeks.
I'm not 100% sure if this was the cause, but my plants' growth resumed
dramatically and there was a lot more pearling.
I don't have a PO4 test kit so I'm just speculating all this. Is it
reasonable to assume my plants are PO4-limited if the growth is this slow
they don't pearl a lot? BTW, I have a 30 g. tank with 96W of lighting and
Flourite substrate.
One reason I'm asking is because I have a huge filamentous algae outbreak.
Don't ask me if it's BBA or hair algae. I don't know, but there seem to be
several types. And if I do in fact have very low PO4, it hasn't done much
against the algae. It currently covers all my plants because of their slow
growth. I have stopped adding micronutrients three weeks ago. I also have an
Fe test kit, and when I was adding micronutrients, the amount of Fe was less
than 0.1 ppm. So what would you recommend? Should I keep dosing the PO4?
Should I restart the micronutrients?
Thank you.
Alex R.