Oh, you actually meant using some kind of resin. No, I do not recommend this in aquaponics. Less is more! Use less chemicals to get more vigorous and healthy plant growth!

pH buffering is not caused by something that acts on the acid you are adding, but it is the acid itself... citric acid will act as a buffer to lowering the pH whereas sulfuric acid will not... again, it has to do with the pKa value and the species caused by the interactions therein...

There must be something else going on for the pH to bounce from 3.5 to 8.0 because that is an ion differential of around 50,000x TIMES which is an enormous amount... enormous... amount... there really are too many things to consider as to why you would observe this, most of which fall under the category of human error, but without specifics there's no way to tell.

The reason plants can take up nutrients when concentrations are low with more moving water is not because nutrient uptake is cumulative... it's a bit more complicated than that