The main reasons are that sulfuric acid is extremely dangerous to handle (the obvious) and constant use accumulates sulfur and can cause the sacrificing of the electrical conductivity.
Same can be said for phosphoric acid albeit very little ppm's in the real world of use. Constant use will induce micro deficiencies. Too much P is not good and is one of the most abused, over used macros on the shelves, witness Bloom foods. A high P food is not really good under most growing circumstances but manufacturers don't care. They will make and sell what they perceive the public wants. IOW, the decision to buy and use a product is often determined by the skill of the salesperson not the efficacy of the product.
Handling - you're telling me 85% phosphoric acid isn't dangerous? We're big boyz, so let's put on our big boy pants, chemical resistant gloves, goggles, and mix up one cup of acid to 1 gallon of water to use to adjust your tank's pH. Easy peasy.... Yes, adjustments should be made after mixing in the nutrients, supplements, etc. Good point. But, not all foods have an acidic influence. Like I said, it's all about trial and error. He'll figure it out. I would not waste my time or money on small quantities of diluted phosphoric acid sold as "pH Down" sourced from brew or hydroponic stores.
I mix fertilizer into a 32 gallon garbage can and using a pump and garden hose feed my trees. Usually don't adjust as I water with rain water but if I do adjust, I mix, adjust and test to a pH of around 5.5-6.0.
When using any concentrated acid, never add water to the acid to dilute.
Thanks, that's the 3rd time we've done the nanny thing.
Having said that, I have a 5 gallon carton of 78% sulfuric acid laying on its side on my workbench so the acid flows well. When I need some I unfold the little white rubber tube, drop it into a jug, tip the back of the box up until done. Most times I'm wearing shorts, clogs, a T-shirt and no gloves. Just took a picture of the box.
A Xmas gift plug - beside that carton is one of the must valuable tools I have on the farm - a Worksharp tool and knife sharpener. Tell Santa you want one, you won't regret it.