My fish tank experienced all kinds of algae problems in past a few
months. I started PMDD practice, now I have virtually zero phosphate,
5-10ppm nitrate, and 15-20ppm potassium. All other types of algae are
now gone, but the hair algae still linger. I thought they can not
survive on zero phosphate?

I then got a few SAE from internet, they look exactly like the true
SAE pictures I found, however, I found them eating fish flakes instead
of hair algae. My Oto work much harder than them.

Since the fish couldn't control hair algae, I'm back to internet to
try to find a solution, now I found Tom Barr's "The Estimative Index".
It says you should have 1ppm phosphate in water volume. Now I am very
confused, isn't PMDD's goal to make phosphate a limiting factor? Am I
missing anything?

Also, when you folks prune stem plants, do you cut them back in half
or even more?

FYI, my tank setup is,
75G, 240W, all flourite substrate, CO2 tank, PH=6.7, KH=4, GH=11,
N=5-10ppm, P=0ppm (0.25-0.5 immediately after water change),
K=15-20ppm, Iron=?, temperature=79F-82F (will be lower in winter), I
don't add extra trace element (because I discovered that whenever I
added flourish, I got hair algae bloom), planted in almost every inch.