Join Date: Feb 2019
Location: Los Alamos, New Mexico
Maryland Guppy covered most of it, but:
HCl is dangerous only when concentrated. It leaves residual chloride, which is actually a minor plant nutrient. Very safe once diluted
It immediately converts bicarbonate to carbonic acid, which then takes a few minutes to break down to dissolved carbon dioxide. Then the dissolved carbon dioxide diffuses out of the water on a time scale of hours, unless you take special measures to aerate it. At first the pH drops dramatically. As the carbonic acid breaks down to carbon dioxide and diffuses out of the tank, the pH bounces back up, though not to its previous value. During this time, most dGH tests will not return reliable results. You have to try a dose, let it equilibrate with air overnight (or with an airstone), and then take your hardness measurement.
If I've done my math right, it takes about 168 milligrams per gallon of the usual 30% HCl sold as muriatic acid to remove a degree of carbonate hardness. That's a little less than three tablespoons to neutralize a degree of carbonate hardness in a 100 gallon tank.
Bump: Adding HCl swaps chloride for bicarbonate on a one-for-one molar basis. Cl weighs 35 grams per mole versus 61 grams per mole for bicarbonate. You'll actually reduce the total dissolved solids on a mg/l basis. But remember that TDS is a rather fuzzy quantity, and what's usually measured on an electrical meter is moles of ions per liter which is then converted to mg/l on the display based on some assumptions the meter manufacturer made about the actual ionic makeup. Since it's a one-for-one swap on a molar basis, the measured value should be practically unchanged.