Use sulfuric acid.
I did the sulfuric for years and still do use it in my veg mix and veg res
my approach has been to reduce the build up of precipitates in the medium reducing the constant need to flush
I want my rockwool water test samples to have an EC that is at least close to what im feeding
if im feeding an EC 1.0 and my rockwool samples are at EC 1.5 then I gotta flush volumes of water thru the cubes to reduce the EC
if im feeding EC 0.8 and 6 weeks into flower my rockwool samples are somewhere between 0.7 and 0.9 im a happy camper and so are my plants
when you look at what minerals cause precipitates or as I like to call it, a build up within the medium, its the chemical reaction between calcium, sulfur, phosphorus that cause the build up
that build up is hard to get flushed so many use sugar water like clearex... yes sugar will add carbs and is fine for microbes in soil or maybe coco but it will also add potential for bacteria to build in an inorganic medium like rockwool so I avoid it
if I could I would lower the P to 50ppm but that would require more sulfur or less K to achieve and I think im already at the bottom of my K needs with 120ppm
I tried flowering with less than 50 ppm and they plants asked for more
I understand the advanced nutrient line has a low P mix .... if I mixed advanced at an EC of 1.0 id only have something like 38ppm of P
I assume that since most people are constantly adding phosphoric acid that they don't see P def from the added P in the acid
from memory.. I think about 30mls of phospuric aicid purchased at hydro store will add 12ppm of P to 30gallon res so if your average person only adds 30ml of ph down in the res within 1 week (everyone does) then theyre getting 50ppm of P
if you happen to have soft water and your source water is ph below 6.0 ... you might see P def using advanced nutrient since you wouldn't be using much acid
am I rambling........