You can use vinigar to bring your ph down... but I wouldn't really recommend it unless you have an acurate ph meter. You could through it to much out of wack easily.
I was told though a rough estimate would be 1 drop of vinigar per liter. I have never tried this method so I can't speak from personal experience, I just went to a pet store and bought some ph down. It was even on sale, so I think it was like 3 bucks total.