Originally Posted by yznj99
Adding a strong acid such as HCl, H2SO4, KHSO4 will not change the pH/KH/CO2 relationship, it will be shifted but it's still there. Using CO2 to moderate pH is not always a good idea especially for DIY CO2 + alkaline water, at the end of CO2 bottle there are often large pH swings. Ideally you should add baking soda plus acid to have a good pH and buffer capacity with or without CO2 addition. However, if you adjust your pH to 6, CO2 can't really crash your pH anymore since it's a very weak acid, at this pH baking soda is not necessary.
In planted tanks, we need a carbon source to get the growth we want in a fast tank. That means CO2. We also need to track how much CO2 we are injecting, and because of the KH-pH relationship, CO2 concentration can be reasonably reckoned by measuring KH and pH and comparing it to the chart such as you find on Chuck Gadd's site, or using his calculator or the formula to do it by hand. This gives us a target pH for a given concentration of CO2.
Drop some other acid in the tank and you shift the zero point to an unknown quantity. Now you would have to jump through some extra hoops to try to figure out where your pH is in relation to KH to read CO2. That means taking a water sample and resting it and testing it for KH and pH at equilibrium, and recording the shift for an assumed 3ppm at equilibrium. Lots of extra trouble that would have to be done every time you do a water change (read: every week).
Now take, for example, hydrochloric acid. Why in the world would you want to add that? It has to be a danger to the fauna at least. We add carbonate, phosphate and sulphate salts to the aquarium already, and what would an acid like this do in that environment? Chloride salts are best avoided in fresh water aquaria.
The conventional wisdom is to add CO2 to our tanks to adjust pH, if it's necessary to adjust it- no other acids or products such as pH Down. This has been the practise for quite some time, and no serious problems with any fauna have been observed from the normal pH shifts associated with CO2 injection as long as you don't turn the water to soda through lack of monitoring, or don't have an equipment malfunction or experience an end-of-tank dump.