Because aquaponics is hydroponics with fish & bacteria, options for adjusting pH are limited.
Primarily people use phosphoric acid, hydrochloric acid, and nitric acid (heavily diluted). A less popular but easy to acquire material is vinegar.
I've tried using it in a very alkaline solution (100 ppm calcium carbonates, 8.4 pH) and noticed that the pH would drop by a small amount before returning to it's original state. Anecdotally, other aquaponic hobbyists have noticed a similar effect that is not replicated by the other acids. What is happening that the pH would lower temporarily and then rise back up over several hours?