Thomas Mann wrote:

Nubile is looking for an advice

Nubile?? I think you mean, newbie.

What kind of chemicals do you use to lower pH in your ponds?

None, zero, nada.

Which of them is suitable for my pond?

Those who insist on using chemicals will say only Muriatic (Hydrochloric)
Acid.

I just do not want to harm fish and plants.

More harm is done to fish by playing with pH than leaving it in the high
8's, but I realize some plants have trouble with the pH that high. I'd
still try to stabilize it in the low 8s using limestone chips, dolomite,
crushed coral, etc., rather than trying to get it lower.
--
derek