Low alkalinity isn't necessarily a problem, depending on what type of fish you have, but it can lead to a problem. In water with a consistently low pH, ammonia will build up due to the lack of alkalinity. Then when you replace the water (during a regular cleaning), the naturally higher pH of the new water mixed with the old water will cause the ammonia to turn toxic. With that in mind, you should try to keep pH above 6 for proper buffering.
Test the water using a basic test kit found at pet stores. While alkalinity is measured in kH (calcium hardness), a pH test interprets the acidity in the water caused by the level of kH. A freshwater aquarium should be above a 6 on a pH test.
Change the water gradually. Change just 20 percent of the water at a time and then retest. If the measurements do not change, then do the same the next day. If after a few days, you do not see any improvement, then you will need to increase alkalinity another way.
Plant live plants that will add carbon dioxide to the tank. Sterilized peat at the bottom of the tank can also increase the alkalinity. Be sure to remove decaying plant pieces immediately and provide adequate lighting. The health of some fish will improve just with the addition of plants.
Cover the tank, so that carbon dioxide is not affected by outside air. The exposure to air in your home can lower the alkalinity of the water in fish tanks.
Place a small airstone in the tank to aerate the water more. This is a piece of porous stone that gradually releases oxygen into the water and is available at most pet stores. The extra agitation should increase alkalinity slightly. You may also need to change the filter cartridges.
Add a 1/2 teaspoon of baking soda to each gallon of new water before you add it to the tank. Let it dissolve completely. Test the water over a week to see whether the baking soda corrected the issues. Increase or decrease the amount of baking soda added to replacement water with each change, until the desired levels are reached.
Use a store-bought aquarium supplement for alkalinity. These usually come in drops and are specialized for freshwater or saltwater tanks. Follow the directions according to the manufacturer's directions, as each supplement is different. Some test kits also come with these corrective additives as part of the kit.