Household baking soda or sodium bicarbonate has many uses in the home aquarium. Most of these uses come back to this chemical's ability to buffer water at a high pH, or to release carbon dioxide in some situations. Both of these properties have uses in aquariums.
Buffering pH
All fish have adapted to the water chemistry of their home waters. For some fish this means they need hard, alkaline water. For example, fish from Africa's Rift Valley lakes or the rivers of Central America, like guppies and swordtails, need water rich in dissolved minerals. Since baking soda contains sodium bicarbonate, it raises pH. It also buffers the water, making it resist changes in pH. For freshwater fish that prefer their water hard and alkaline, baking soda is an easy fix.
Guidelines For Raising pH and Hardness
If you want to raise the pH of an existing aquarium, you need to use caution. Sudden changes in pH—even changes towards more ideal conditions—can shock a fish and cause more problems than the incorrect pH. To alter pH, add about three-quarters of a teaspoon of baking soda per 10 gallons of aquarium water. Let the water sit for an hour, than test the pH. Once you've upped the pH by .2, stop for the day. Changes of greater magnitude will cause problems for your fish.
Carbon Dioxide Reactors
Additionally, baking soda can help power DIY carbon dioxide reactors. Planted aquariums often need extra carbon dioxide for the plants to thrive. Aquarium hobbyists with a DIY inclination can make their own carbon dioxide reactors to up the carbon dioxide levels. Yeast "power" these reactors by producing carbon dioxide. Usually the nutrient medium includes some baking soda—and other nutrients like sugar and protein powder—to buffer the environment and provide a source of raw carbon dioxide.
Brine Shrimp
Additionally, baking soda can be used to help culture brine shrimp. Brine shrimp make a great food for many aquarium fish, and part of their appeal is how easily they can be cultured. However, brine shrimp come from very hard, alkaline water. Mixing baking soda into these shrimps' water can help reproduce those conditions and improve hatching yield. Add about a quarter of a teaspoon of baking soda per liter of water when mixing a hatching solution for brine shrimp, to buffer the pH and increase water hardness.