Basic phosphoric acid is great, and most common.. The reason being, phosphoric acid will react/neutralize in solution to form things like potassium dihydrogen phosphate/dipotassium hydrogen phosphate.. These types of salts in solution will 'buffer' the solution against pH drift very well at pH 6-7.. (eg loosely speaking, a solution buffered well at pH 6-7 will take fewer drops of pH down to go from 9.0 to 7.0 than it would to go from 7.0 to 6.
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So essentially, the more 'neutralized' phosphoric acid you have in the tank, the better buffered it will be, and the more stable at pH 6-7..
Phosphates were mentioned.. Some brands may be souped up with neutralized compounds to increase the buffering capacity, but you could easily make your own phosphate buffers if you can get some potassium/sodium hydroxide, and standard phosphoric acid pH down..
For the quanities/concentrations we'd require this can be done safely, but these things can react VIOLENTLY together, producing heat and splahing everywhere.. And they can be VERY corrosive.. Even more so when warm/hot..