I've used vinegar in the past, best to get something where it states on the bottle the percentage and type of acid. Acetic acid at 4.5% is the one I've used in the past.

I always used it in the feed water, like 10ml per litre. If you have a large adjustment to make you should do it over a few days rather than all at once.

Always go VERY SLOWLY and use a decent PH meter. Take readings an hour or so after mixing with the water.