Question
I have a fifteen hundred gal., concrete pond.-very alkaline; very expensive using ph-down.can I use muriatic acid in place of?

Answer
Absolutely.  Muriatic acid is another name for hydrochloric acid (HCl).  They usually sell it at pond stores under muriatic acid.  HCl is a simple inorganic acid.  You can mix some with water, add that, recheck the pH in a day and repeat if needed.

Is the pH of the pond high because the concrete is leaching?  If so, I suggest draining the pond and either sealing the concrete with one of the pond paints or acid washing it with muriatic acid to leach it out.  The HCl will lower the pH but it will not lower the hardness which must be pretty high as well.  If your tap water is lower in pH and hardness, water changes will help bring the pH and hardness down.

Good luck!