Q: Adjusting pH levels

Hi all,
Nubile is looking for an advice
What kind of chemicals do you use to lower pH in your ponds?
Yes, yes, I know, you buy stuff in local pond store.
Unfortunately, I do not have that kind of shop close enough.
However, I do have chemical store in my neighbourhood which provides
chemically clean acids.
Which of them is suitable for my pond?
I just do not want to harm fish and plants.
Thanx,
Tom