Instructions
Add peat moss or driftwood to the tank. Both release humic acid and tannic acid as they decompose, which will slowly lower the alkalinity of tank water. You can add peat moss to your tank's filter to make sure the acids are mixed throughout the tank. You will have to experiment with the amount of peat moss or driftwood you add in order to achieve the right balance.
Supplement your tank with carbon dioxide. Aquarium owners who grow live plants often add carbon dioxide to the water, because plants take in carbon dioxide and release oxygen as they grow. Carbon dioxide dissolved in water creates carbonic acid, which lowers alkalinity.
Mix your tank's water with reverse osmosis water. Reverse osmosis filters purify water and remove nearly all of its dissolved minerals. Fish and plants cannot live in pure reverse osmosis water, so you must either blend the filtered water with tap water to reach your target pH, or add minerals back to the water. Aquarium stores sell products that when mixed with reverse osmosis water help aquarium owners achieve a neutral, stable pH measurement.
Use pH-reducing chemicals. Aquarium stores sell commercial products that you can add to tank water to lower the pH. Without removing the dissolved minerals in the water, however, the tank water will tend to bounce back to a higher pH.
How to Lower the Alkalinity in an Aquarium
Before you try to lower the alkalinity of your aquarium water, it helps to know a little bit about water chemistry. The pH scale measures the acidity or alkalinity of water, on a scale of 1 (very acid) to 14 (very alkaline.) A measurement of 7 is neutral and is considered safe for most aquarium fish. Usually water that is alkaline also has a high amount of dissolved minerals, which make the pH difficult to lower. To bring alkalinity down, you must remove those minerals or introduce an acid to the tank.