Water hardness Is Hard Water Bad For Planted Aquariums?

Is Hard Water Bad For Planted Aquariums?

Hard water and high pH makes it difficult for many aquatic plants to absorb important nutrients, such as iron and manganese. However, amazon swords, water wisteria, hygrophila, cryptocoryne and other species can grow well in hard water because they demand fewer insoluble nutrients.

This article will explore the affects of water hardness on your planted aquarium. Plus, how to successfully grow aquatic plants in hard water.

General hardness (GH) test
General hardness (GH) test

Why is hard water bad for aquarium plants?

Hard water water itself is not bad for aquarium plants. What plants have difficulty with is a high pH, which is caused by a high concentration of carbonate hardness (KH).

KH and general hardness (GH) are the two types of water hardness we are most concerned about when talking about planted tanks. Let’s talk about the role each of these plays in our aquariums:

General hardness (GH) measures calcium and magnesium ions

General hardness is mostly important for the health of your fish. Specifically, studies show that fish species have higher growth rates and reproduction rates when they are in ideal water hardness.

Some species prefer higher concentrations that others due to how calcium and magnesium affects their metabolism and osmoregulation.

Plants, on the other hand, are pretty much unaffected by the GH concentrations found in freshwater aquariums.

Carbonate hardness (KH) measures carbonate and bicarbonate ions

Carbonates and bicarbonates also have little direct impact on plant health and growth. But they do play an important role in maintaining a stable pH, which is crucial for plant health.

Specifically, a higher concentration of carbonates and bicarbonates will cause your water to have a greater resistance to pH swings. This is because they release or absorb excess hydrogen or hydroxyl ions.

pH is important for plants because if affects whether or not important nutrients can dissolve into the water column.

If they cannot dissolve, they are consider insoluble and plants are unable to absorb them and benefit from them. Some nutrients, such as calcium and manganese, start to become insoluble as the pH reaches 8.0 or higher.

This means that certain plants species may suffer if they demand higher concentrations of these nutrients than other species.

Why does pH affect nutrient solubility?

When the pH gets too low, things like aluminum, iron and manganese become more soluble.

Iron and manganese are important for aquatic plants but only in small quantities. Excess amounts can be toxic by stopping the growth of root systems.

Nutrient solubility (Source: Bluedale)
Nutrient solubility (Source: Bluedale)

As you can see from the chart above from Bluedale, a mostly neutral pH allows most macro and micronutrients to be soluble and available for plant use.

Ideal pH for planted aquarium

A pH of 6.5 to 7.5 is an ideal range for most aquatic plants to grow. Most macro and micronutrients that are important for plant growth become soluble in this range. Meaning, plants are able to absorb them.

Achieving this pH range can be tricky for many fishkeepers because most of us use our local tap water as the water source for our tanks. Your tap water hardness will vary depending on where you live and where your city chooses to source it’s water from.

For me, in Calgary, m tap water has a pH of 8.2, dKH of 7 and dGH of 13. Although these figures changes slightly depending on the season and other local water supply implications.

The best thing to do is test your tap water before setting up a new aquarium so you can determine which plants and fish species will thrive in your local conditions.

How to reduce water hardness

I generally suggest trying to buy plants and fish that match your water parameters instead of trying to change your water. This is because it can be difficult and time consuming to maintain precise water parameters when they are different from your tap water.

But there are a few methods for reducing your water hardness if you choose to.

You can lower your water hardness by soaking tap water in peat moss, Indian almond leaves or driftwood. You could also try using reverse-osmosis systems for more precise results.

Ideal KH and GH for planted aquarium

The GH doesn’t really matter for plants. Therefore, it’s mostly important that you buy fish that are suitable for your water’s GH.

It’s useful to have a dKH of at least 5 to resist pH fluctuations.

Is softening water bad for planted aquariums?

The main reason soft water might be bad for planted tanks is if the KH levels are solow they allow pH swings to occur.

pH swings occur when there is a change between the concentration of hydrogen and hydroxyl ions. The more hydrogen ions there are, the more acidic your water will be and the lower your pH will be. More hydroxyl ions will cause the pH to increase.

The concentration of these ions will changes with many factors, such as decaying plant foliage, increased CO2, added fish waste and more.

These ions tend to fluctuate more than you might think. But the reason the pH doesn’t change is because your KH buffers against it.

Most reliable hard water aquatic plants

Although a pH between 6.5-7.5 is optimal for most aquatic plants, there are some species that can grow well in hard water conditions with a high pH.

Hygrophila species

I’ve had success growing both water wisteria and hygrophila polysperma in 8.2 pH water. Both of these species are known to be fast growers that don’t require much light, CO2 or fertilizer supplementation – although they all help.

In my tank, neither grow prolifically. But they are among the easiest plants I’ve had success with in my hard water conditions.

Cryptocoryne species

Cryptocoryne species are regarded by many as being exceptionally easy growers. I’ve had success growing cryptocoryne lucens variety, which I think look quite nice.

There are many cryptocoryne species to choose from, all of which are quite easy to maintain.

Popular varieties include:

  • Cryptocoryne Wendtii ‘Green’
  • Cryptocoryne Wendtii ‘Brown’
  • Cryptocoryne Wendtii ‘Red’
  • Cryptocoryne Undulata
  • Cryptocoryne Lucens
  • Cryptocoryne Parva

Staurogyne repens

This species may not be on everyone’s list but I’ve added it because I’ve had success growing it. It’s often considered a good beginner carpeting plant because it grows quite lush and can be trimmed to create a pleasing carpet affect.

Anubias species

Anubias are popular because they require less light than many plant species. They are a mid-ground plant that are often found in partially-shaded areas in front of over-hanging background plants.

Popular varieties include:

  • Anubias Barteri
  • Anubias Barteri Nana
  • Anubias Barteri Nana ‘Petite’
  • Anubias Barteri ‘Coffeefolia’

Amazon swords

Amazon swords are very resilient, beginner-friendly plants that grow rapidly in many water conditions. They produce a beautiful, tall green leaves.

Ryan Ferguson

Founder, Rooted Tank

Ryan Ferguson, the founder of Rooted Tank, started fishkeeping in 2019. He has continued to level-up his planted aquarium skills and wanted to share his journey and knowledge with other aquarists.



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