Another consideration of KH is that you can generally safely add the buffers (both freshwater and saltwater) that effect KH without sudden changes in chemistry (unless your freshwater KH is under 50 ppm already), unlike a direct ph or GH change. Maintaining these KH buffers keeps your tanks pH form drastic swings which can be deadly.
Baking Soda (Sodium Bi-Carbonate HCO3-), is often used for KH, Sodium Bi-Carbonate will buffer at 8.0 to 8.2. Just a little Sodium Carbonate will absorb free H+ ions, and this causes alkalinity (which is the lack of H+ ions).
To stop the Sodium Carbonate ions from consuming too much H+ and to keep a pH of 7.0 we need to restrict the amount of Baking Soda used, as it is always looking for H+ ions to consume. This is why I prefer using products that not only contain sodium carbonates (or sodium bi carbonates), but the proper ratios of other minor elements such as Calcium and Magnesium. Sea Chem Buffer (Marine OR Freshwater) and to a lesser degree, aragonite and Mineral Blocks are examples.
Sea Chem Buffer can be safely used for raising kH (& pH) in freshwater as well or you can use Sea Chem Alkaline Buffer as both are preferable and safer than baking soda, especially in community aquariums where baking soda can change pH too quickly (also baking soda maintains a more stable KH). The added calcium and other elements keep a more stable KH/GH and add necessary trace and minor elements (of course, use in moderation in freshwater). This article will explain a little more about the need for other elements besides Baking Soda (HCO3): "Proper Osmotic Function; Electrolytes".
Wonder Shells are an excellent compliment for raising Calcium, magnesium, and electrolyte levels quickly when used with aragonite or even Baking Soda, however by themselves they do not raise KH much (they are great for Calcium and electrolytes though and for maintaining KH).
Sea Chem Buffers or similar products are better for pH/kH control in livebearer and African cichlids aquariums or a combination of Marine Buffer and Wonder Shells. Before I over promote Wonder Shells, these can aid in proper water management, but they are not a magic bullet in ANY aquarium for poor aquarium husbandry such as mulm build up under gravel or decorations.
Aragonite or crushed coral is sometimes employed for KH and GH stabilization, however aragonite and crushed coral (as with Wonder Shells) only stabilize KH and should not be used in place of a true KH buffer such as Sea Chem Alkaline Buffer .
I do not recommend aragonite for soft water or general aquaria.
Aragonite is good at stabilizing a higher kH of around 240 ppm or more, which is the minimum KH (alkalinity) needed for Marine Aquariums, but does not respond to changes rapidly enough at lower kH levels. Even in marine aquariums with aragonite, this may not always be enough to maintain a proper KH (alkalinity) level, especially in tanks high bio loads and without adequate water changes (even skimming can remove some elements).
In lower pH community tanks (6.8 to 7.4) KH buffers such as Sea Chem Alkaline Buffer are still important, however I like to counter these with natural lower pH “buffers” such as Almond Leaves , Peat, and/or Mango/Drift Wood. Buffering your freshwater aquarium is especially important if you have plants fed by CO2 which will raise pH during peak growth times, and there is scientific evidence that GH plays a role here as well; please see this article for more about this subject: “AQUARIUM PLANTS; see PROPER NUTRIENTS ”
Back to baking soda, this is an old stand by method based on the fact that baking soda does raise pH and kH, the problem is there is much new research to show that calcium, magnesium, electrolytes and Redox play a more important role in aquatic chemistry than just pH or basic kH alone (which is another reason good old fashioned water changes often help improve fish health). Unfortunately the aquatics hobby is full of aquarist (especially in forums or uninformed LFS) that regurgitate this old information without checking the facts. If Baking Soda is used, I recommend using Wonder Shells to added needed calcium and other important electrolytes.
If you have a very unstable KH level (drops rapidly), look into causes such as a large amount of decomposing organic material. The more organic break down (de-nitrification), the more acids produced. Some filters if not cleaned regularly can cause this; including canister, UGF, and Wet/Dry.
For a really informative (and scientific) article about the relation of KH (Buffers) and pH, please follow this link:
pH of buffer - Henderson-Hasselbalch equation
I did not write this altho I agree.