This is what i've been using in my reservoir for years(battery electrolyte-sulphuric acid),a little goes a long way so you use it sparingly,and use caution when you're handling it as it will burn.
it's the most stable ph down that i have ever used,stays stable for days with only a slight fluctuation of maybe several tenth's of points...at around $10.00 a quart from a local auto parts store, it can last a couple of years,and the plants really like the added sulphur.
vinegar,citric acid are unstable and do not work well in a chemical nutrient mix.
they are food products and decompose in the water as it breaks down creating muck and gunk that'll float around or settle on the bottom of the tank and the ph will really start going out of whack which will also contaminate the nutrients.
and commercial hydro ph down is too way overpriced.
I know this from personal experience.