Join Date: Feb 2019
Location: Los Alamos, New Mexico
Distilled vinegar would not be my choice of an acidifier. It's not that strong an acid, it smells -- which means it won't stay permanently in the tank -- and some organisms will actually consume it over time.
On the other hand, it's cheap and safe to work with.
If you are comfortable working with strong acids, some muriatic acid (hydrochloric acid) from your hardware store is not too expensive, is much more powerful, and goes to chloride in your tank, which is harmless, odorless, and not much consumed by tank residents. I used hydrochloric acid routinely to acidify my rather alkaline local tap water back when I was keeping discus. In fact, I still have some of the bottle after twenty years, which I occasionally use for other purposes.
A middle ground is sodium bisulfate, which is sold for use in pools and spas to bring pH down. This assumes you can tolerate a bit more sodium in your tank. Sulfate is pretty innocuous, being a plant nutrient that is not consumed rapidly.
If you decide to acidify, regardless of the acid chosen, don't overdo it. You shouldn' try to bring the pH below about 7.5 in water equilibrated with air. (Lowering it further with carbon dioxide injection is fine.) Also, if you add acid to a bucket of water until its pH is 7.5, you'll come back the next day and it will be back up to 8 as the water re-equilibrates with the air. It will take some experimenting to find the right dose to get it down to 7.5 for good, and you'll get it downright acid before it equilibrates to 7.5. So you probably don't want to acidify as you're adding to your tank unless you do frequent small water changes. Acidify in an aerated bucket and let sit overnight before doing the water change.