It won't impact anything else except pH, which it lowers a lot. Aeration will slowly bring it back up.
Here's a copy and past of an old post of mine for a guy who wanted to use it to drop the alk from 13 to 7 dKH in 13 gallons of salt water.:
Yes, you just add the acid. Be very careful with straight muriatic acid. It can easily burn you, especially your eyes.
The "acidity" (that being essentially negative alkalinity) of normal (not low fume type) muriatic acid straight from the bottle is about 11,000 meq/L.
So adding 1/11,000 of the water volume as this acid will drop alkalinity by 1 meq/l (2.8 dKH).
You want a drop of 13-7 = 6 dKH, or 2.1 times that amount, so you'd add 1/5,100th of the water volume.
13 gallons ~ 49 L
1/5100 of 49 L = 9.6 mL
So I'd add 5 ml and stir well for a few minutes and see what alkalinity you get. Then dose again assuming it seems on the right track.
You'll need to aerate well after adding the acid to blow off the excess CO2 and bring up the pH.
