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Is harder better, faster, stronger?

The importance of water hardness in your freshwater aquarium

Water hardness GH and KH in freshwater planted aquariums


Water parameters

Natural bodies of water have specific chemical properties that reflect the geological features of the area. As water falls from the sky, flows over land or passes through the ground, it acquires gases, minerals and other particles from the environment. These constituents, among other properties, collectively determine an ecosystem’s water specifications. Water parameters are the values that quantify these specifications, and they may vary greatly from one ecosystem to the next. Because plants and animals are physiologically adapted to the conditions of their natural habitats, it’s important to replicate the parameters as closely as possible to maintain a healthy aquarium. Water parameters are a good indicator of an aquatic community’s window of tolerance.

Water hardness

Water hardness refers to the dissolved mineral content of a given solution, and it’s one of the most important parameters of freshwater tanks. Different species prefer specific ranges of water hardness. In aquarium-keeping, we focus primarily on carbonate, calcium and magnesium. The more minerals a solution has, the harder it is, while low mineral content means softer water. To measure total hardness we look at two parameters:

1) carbonate hardness (KH): the measure of a solution’s carbonate and bicarbonate content, which is an important chemical in aquatic biology and closely related to pH and CO2
2) general hardness (GH): the measure of dissolved calcium, magnesium and trace amounts of other minerals

In freshwater tanks, we test for GH because calcium and magnesium are similar in depletion (not the case in reef aquaria). The units of measure are parts per million (ppm) or degrees of hardness (dKH and dGH).

pH and alkalinity

pH, another key parameter, is a scale that indicates how acidic or basic a solution is. A pH greater than 7 is basic, or alkaline, while a pH lower than 7 is acidic. Carbonate hardness makes water more basic and acts as a buffer to protect against pH instability, which means mineralized water is more able to resist changes in pH.

Directly related is alkalinity, the measure of a solution’s ability to neutralize an acid. Various minerals contribute to alkalinity, but carbonate is the most significant. Carbonate binds to acids as they appear, thus neutralizing them.

This is where many people typically lose the plot: carbonate hardness and alkalinity arenot synonymous, though you’ll hear aquarists use the terms interchangeably. Alkalinity is a measure of the water’s buffering capacity, and while carbonate is the greatest factor of alkalinity, it’s not the only one.

Water Hardness (KH and GH) in freshwater planted aquariums | Aqua Lab Aquaria

General and carbonate hardness in the aquarium

GH and KH are both needed in a system for different reasons. GH is vital to a fish’s biological functions. Although KH doesn’t affect fish in the same way, its capacity to stabilize pH is reason for regular monitoring.

That’s because fluctuations in pH can be fatal to fish. Acidosis and alkalosis are common aquarium diseases that afflict fish in water with reduced or elevated levels of pH. Fish are happiest in steady conditions; they are especially intolerant to rapid changes in pH, even if it’s within their preferred range. Because water enters fish at the cellular level through osmosis, a change in pH changes the pH of their blood too. Regulating internal pH takes time and energy, which is why sudden changes and unstable water quality can be dangerous. We aim to avoid subjecting our fish to a difference in pH of more than 0.3 in 24 hours.

Depletion of minerals

Monitoring is also necessary because KH and GH are constantly depleted throughout the lifecycle of an aquarium. GH is regularly removed from the water through its use and absorption by animals. Biological processes such as the decomposition of organic waste or animal and plant respiration neutralize KH by introducing acids to the system. Additionally, CO2 reacts with water to form carbonic acid, and reactions like biofiltration consume carbonates. These occurrences all have the potential to lower pH.

Ways to achieve balance

To lower KH in your tank, the simplest and most effective way is to dilute with a source that has no KH, in effect removing carbonate compounds. We have a nifty water change calculator here to help you figure out how much water to replace when attempting to lower levels of any compound. Adding products like Acid Buffer™, Super Peat or natural acid-producing substances like almond leaves or cholla wood is another way to lower KH by neutralizing carbonates and bicarbonates.

Raising KH or GH requires dosing with compounds that are added directly to the aquarium or mixed with water to form a solution. We recommend Alkaline Buffer™ for KH and Remineraliz-P for GH. Yet it’s important to remember that slightly acidic water is preferable in planted aquaria. A significant substrate layer of nutrient-rich aqua soil, like Amazonia, offers acid-buffering capacity, so 0 dKH is acceptable in these cases.

Lowering KH before pH

Because of its role as a buffer, KH must first be addressed before pH can be adjusted. Attempts to add acids to lower pH without first removing KH from the water will be ineffective because the acids will neutralize, and the pH will remain unchanged. However, once the buffering capacity of the water is reached, adding more acids can cause a drastic dip in pH and pose a potentially greater hazard.

Water changes

Water changes are necessary to replenish depleted minerals and remove toxins. The new water should have correct amounts of KH and GH. Although tap water contains these minerals, it may not have the right levels. Plus, it can contain loads of other stuff and change seasonally or by municipality, so it’s not recommended in most cases. If tap water is your only option, testing it is always a good idea. Knowing exactly what is added to your system is a safeguard against exposing your aquatic life to certain hazardous conditions.

RO/DI water is the best solution. Due to its purity, it lacks minerals, so be sure to remineralize the water accordingly.

Topping off

Water in aquariums typically evaporates, leaving minerals behind. As the water level drops, hardness increases because the same amount of minerals is contained in less water. Topping off with RO/DI or distilled water will bring the water back to the level of hardness after the most recent water change. Adding tap water or remineralized RO/DI will only further increase the mineral concentration.


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