VERY SLOWLY....if you have fish in the water....don't ask why I said that first....

I use muatic acid (for swimming pools).
It takes very little....no more then a 1/4 cup of acid to a 1,000 gals. That little, will show a slight ph drop when tested. I wouldn't do it more then twice a day.....8 to 10 hours apart.

I use a BIG plastic drink cup, with a small hole in the bottom....add some water, then a little acid, top off the cup with more water. It 'slowly' drips into the water and is mixed in.