1 gallon of HCl @ 38% concentration weighs 9.901 pounds.
1 mole of HCl weighs 36.5 grams.

Weight conversion:
9.9 lbs * 2.2 kg/lbs = 21.8 kg

HCl weight conversion:
21.8 kg * 38% = 8.3 kg
8.3 kg * (1000 g / 1 kg) = 8300 g of HCl

Grams of HCl per Liter:
(8300 g / gallon) * (1 gallon / 3.78 Liters) = 2196 g / L [which is 2,196,000 PPM for the curious]

Moles per Liter:
2196 g * (1 mol HCl / 36.5 g) = 60 moles of HCl per Liter

60 moles of HCl per Liter
HCl is a strong acid, meaning that it will disassociate 100% in a solution. We also know the pH of a solution of 7 is 10^-7 moles of H+ per Liter. Therefore, 1 mole of HCl will lower 10,000,000 Liters of a solution of pH 8 or high to roughly pH 7 (roughly meaning +- 0.1 pH).

So, for 4000 Liters we would need roughly...

(1 mol / 10000000 L) / (4000 L) = 0.0004 moles of HCl

(0.0004 moles of HCl / 60 moles of HCl) = 6.7 x 10^-6 % of 1 Liter or 6.7 mL of the solution per 4000 Liters (roughly 1000 gallons)

So, for every 1000 gallons add 6.7 mL of 38% HCl (Muriatic Acid). If your local store only carries 10% use about 25 mL of 10% HCl (Muriatic Acid).
Please review this acid safety before beginning any work unless you are a professional: http://www.flinnsci.com/sections/safety ... TY_ART.pdf

SAFETY GOGGLES AND GLOVES ARE A MUST. I do not work without these, at a minimum (depends what I am doing).

10% is far safer and more commonly available, but still observe safety and caution. Always add acid to water and never the other way around (even for low concentrations). NEVER work indoors with this stuff unless you have a fully equipped laboratory to work in (again, even with low concentrations for safeties sake).

SPECIAL NOTES
A buffer occurs when a molecule has speciation at different pH values. This happens because the molecule might grab 1 hydrogen ion at one pH and as you move through the pH levels it might grab 2, or 3, or 4. The amount of speciation a molecule will exhibit is indicated by it's pKa value. This can be looked up and calculated easily, but you would have to know EVERYTHING in your water. A quicker approach would be to put just enough in the water to get the expected pH value and wait 24 hours. After everything settles, take another reading. From there you can adjust accordingly. For future reference, you will know what your system roughly takes to adjust if you have to do a full water change or you can calculate the volume ratios and have that on hand!

ECONOMICAL NOTES
$5 worth of this stuff will treat hundreds of millions of gallons.

DANGER TO FISH?
Obviously, HCl will leave Chlorine in your water, but the toxicity for fish is millions of times higher (in this case 10 million) than the concentration of Chlorine we are adding. Also, it will volatilize OUT OF THE WATER within a few days. No worries there.

Sample MSDS
http://www.brown.edu/Departments/Visual ... icacid.pdf
http://www.sciencelab.com/msds.php?msdsId=9924285
It is unlikely there will be anything extra in your solution because HCl is VERY reactive. However, always verify with your supplier and check the MSDS or have us check it for you if you have any doubts.