Carbonate Hardness in your Saltwater Aquarium

June 27, 2008 · Filed Under Aquarium Care and Maintenance · 2 Comments 

Hardness is defined as the amount of dissolved minerals such as calcium, magnesium, iron, and zinc. In most water it consist mainly of calcium and magnesium salts, with trace elements of other metals. If your aquarium water has a high concentration of minerals, it is said to be hard. If it has low levels, it is soft water.

Negative molecules that are typically found in saltwater that act as buffers are called carbonates and bicarbonates. There is a specific kind we call carbonate hardness that you need to measure in your saltwater aquarium.

Checking carbonate hardness (alkalinity)

Clownfish in Anemone Measuring carbonate hardness is referred to as measuring the alkalinity of the water, but this different to the pH measurement that is alkaline. Carbonate hardness affect pH, but you’re not measuring pH. Carbonate hardness can be measured with a test kit. It may be called as alkalinity test kit. It is usually reported in milli-equivalents per liter (meq/L). The level in your aquarium should be greater than 3.0 meq/L.

How to adjust carbonate hardness (alkalinity)


The if the readings are below the recommended level, adjustments need to be made. Your pH will be decreasing as well if alkalinity is not in the normal range.

There is three methods for adjusting your pH and are also used to increase the carbonate hardness (alkalinity).

  • Water change

  • Remove the old water and replace with fresh buffed water saltwater the pH level will adjust too. This is the first step. Click here for details on changing your saltwater water.

    Test the water again after about a day. If the carbonate hardness is still low, follow the next two steps.

  • Use a commercially manufactured aquarium buffer.

  • It’s available at most aquarium dealers. It buffers the aquarium water if you use it properly. Make sure you follow the manufacturer’s instructions.

  • Kalkwasser

  • Add a solution of calcium hydroxide – kalkwasser. This will increase the buffering capacity (alkalinity) of your water.

Complete guide to maintenance and care of your saltwater aquarium.

Recommended Water Testing Kits by Saltwater Aquarium Guide

Text and Photo Source: Saltwater Aquarium Online Guide