Sodium chloride is the same chemical as table salt, rock salt, “aquarium salt”, and salt sold for recharging whole house water softeners. Sodium chloride is only one of the many elements that make up TDS, and is not usually an element we try to add in significant quantities. Generally the primary ingredients we look for in a product to increase TDS are magnesium, calcium, carbonate, and bicarbonate. Seachem is one company that does a good job creating buffers (increase or decrease the pH) and salts for freshwater aquariums that can help you recreate as close as possible the water conditions of several unique freshwater environments.
How much of a product you will use for your system will depend on the hardness of your tap water. Seachem provides a chart on many of their products that give you examples of how to mix their products, and the ratios to use, but these charts are based on you starting with distilled, reverse osmosis, or deionized water, all waters with very little or no hardness. You will need to adjust the amount you add based on what is in your source water.
If you are adventurous, you can find the raw ingredients at the grocery store, hardware store, or pool supply store. Baking soda is sodium bicarbonate, soda ash or swimming pool pH increaser is sodium carbonate, and swimming pool calcium increaser (or Dow Flake) is calcium chloride.
Whenever you add chemicals to your freshwater system, you should always first add the chemical to a container of water and then mix thoroughly until completely dissolved. Some of these chemicals can get very hot when initially mixed with water and should be stirred quickly while mixing. Pour the mix slowly into a high flow area in the aquarium. Dumping raw chemicals straight into the system can harm or kill aquatic life.
pH is the measure of whether a solution is acidic or alkaline (basic). pH is measured on a scale of 0 to 14, 7 being neutral and the pH of pure water at 77°F (25°C). A pH below 7 is considered acidic and pH over 7 is considered alkaline or basic. pH measures the concentration of hydrogen ions in the solution. A low pH represents a high concentration of hydrogen ions, and a high pH has a low concentration of hydrogen ions. Each point on the scale represents a 10 fold increase or decrease from the next point in the concentration of hydrogen ions. Chemicals that are used to raise or lower the pH are called buffers.
Most freshwater rivers, lakes, and streams fall between 4.0 and 9.2. Fish that come from water that has a pH as high as 9.2 should not be kept in water with a pH range of 4.5 to 6.8 and vise versa.
Fish that have evolved in water that has a high pH will usually not live long in water that is acidic. Fish that are kept in a pH range that is not considered normal for that species will be under physical stress which lowers their bodies immune system. Fish that are kept in the wrong pH range often have health problems. Freshwater that is soft and has a high organic load is often very acidic. The black water (tea color) of the Rio Negro, in the Amazon Rainforest of Brazil, is one example of very soft and acidic water, with a pH value of less than 4.5 much of the year. Fish that live in the Rio Negro have evolved to tolerate such a low mineral content, and are more efficient at extracting salt from the environment than most fish. Fish that come from such waters often have a limited tolerance for water that has a lot of dissolved salts. Lake Tanganyika (in Central East Africa) is at the other extreme, its water is often measured at a pH of 9.2 or higher. Fish from Lake Tanganyika are adapted to living in hard alkaline water, and often do poorly when kept in soft acidic water.