Nail a good trimming if you see any GHA appear asap.
Water change, spot treat the infested plants/area with excel, then wait a few mon, turn filter back on etc. Tweak the CO2 a little bit higher and watch very closely when you do this.
Adding more PO4 does not cause issues, near as folks can tell.
Some do this to prevent GSA, I'm not so sure it does.
Uptake of nutrients/CO2 is not linear.
Many assume that is it and many like to think themselves as very knowledgeable about dosing ferts. I really do not fuss much with ferts, but light and CO2 instead. that said, I know the basics and how plants respond over time to ferts.
It's sort of an intermediate level for many planted hobbyists, some folks need to get it out of their system and they run from each fert to the next and never see the holistic picture. Most of the folks do this, myself included years ago, but eventually you realize it does not matter much.
Thi is something many aquarist have to go down and experience themselves, does not matter what I or anyone else tells them. But they can cause a mess for newbies in the interm. Most figure it out later fortunately and then have both experience and knowledge to help new folks.
It's good to learn from experience as long as it's not your own, but many of us are not wired that way or that accepting. (Which is a good trait also!)
the CO2/HCO3 thing with people is pretty cool if you think about it. Like aquatic systems and plants, the gas phases for CO2 and O2 MUST go into/out of the aqueous solution for plants and animals/people.
If you change the pH of the blood/plasma etc with something other than CO2/HCO3........this can cause issues, same deal with planted tanks and CO2.
Now if the KH is entirely all HCO3.......then you are set.
There's a cool way to get around this, use a gas membrane with a reference pure KH HCO3 solution in it. Then a pH probe. the gas will go across the membrane, but not the acids, non carbonate KH/HCO3 etc.
This can be done for measuring Human CO2/HCO3 is such situations etc. I would assume that woud be the best method to avoid issues in the blood and other non carbonate KH.
Still, healthy patient vs healthy plant/livestock, you can see what works and if you adjust and observe SLOWLY. I have 3-4 different CO2 ppm's in each tank I have.
No one tank is the same as the other. Why? Not sure, have some ideas, but every tank is unique where than D range is.
CO2 also have a similar pattern.
Here's a good article on light and CO2: