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- 1). Use a hardness test kit to determine the hardness level of your aquarium water. Follow the manufacturer's instructions. Some types require you to dip a test strip into the water; others require you to collect a sample of water and mix it with a few drops of the test liquid.
- 2). Increase the water hardness if the water is too soft. You can add calcium carbonate into the water. Items such as limestone, crushed marble and lime sand can all release calcium carbonate and are widely available in fish stores.
- 3). Decrease the water hardness if it is too hard. Run tap water through a tap water filter to remove ions in the water, then pour it into the salt water aquarium.
- 4). Test the water for hardness using the test kit to check that it has reached the desired level. Add more calcium carbonate or de-ionized water to further adjust the water if necessary.
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