Algae is taking off because your plants are struggling. This is more of a plant health issue than a light issue.
Many of my leaves are turning brown/dark brown and wilting away. I've read this could be excessive phosphates or nitrates, but they have never tested over 2ppm or 20ppm. Could it be the potassium from the flourish potassium and iron doing it? I can't test that, so I've just been dosing according to instructions.
No, too much nutrients doesnt cause algae. Lack of something will.
Again this is all about plant health. Healthy plants dont get algae. A clean tank full of healthy plants doesnt either.
Focus on plant health rather than trying to "fight" algae. Get the plants right, maintain clean conditions, and the algae will take care of itself.
Of course in the meantime a little Excel/H2O2 may be called for, along with manually removing as much as you can. Maintain super clean conditions, remove any decaying or dead plant matter, keep filters clean, regular 50% water changes, etc etc.
Id be careful about reducing light too much. You want to continue to give the plants plenty. Having said that, 2 t5HOs sitting right on top of a 20H is probably too much. One bulb should be fine until you can raise the fixture a little bit.
The stems degrading at the substrate is a nutrient deficient issue. You said you ran out of P? Could be as simple as that.
How low is too low for PH?
When the fish start gasping at the surface.
This is due to having too much CO2, which starts to affect respiration. It's not directly related to the pH level. We are just using PH to gauge the CO2.
Sit some tank water out in a bowl for 24 hours to let all the co2 de-gas. Check the PH.
This is your water with no co2. Compare to the tank's PH with the co2 running.
Shoot for a full 1 point drop (at least), preferably by the time lights come on, or fairly close to it.
This is where you'll have to dial in your rate of injection vs how early the co2 kicks on.
For example in my 120 gal, I can drop the PH 1 point in about 20 minutes. But then it becomes way too much after a couple of hours. So...I run a lower bubble rate and turn it on earlier, couple hours before the lights.
CO2 concentration doesnt climb indefinitely. Whatever rate you're injecting will top out at some point and stay there. So play with the bubble rate and when it kicks on.
Not sure what kind of PH test you're using, but a $20 digital PH pen is the best investment you can make (along with some calibration fluid to go with it).