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  • Writer's pictureGautam

How alkalinity of water affect algae and plant growth in an aquascape (or a planted aquarium)

I'd like to preface this blog with a disclaimer. While my co-author, Anushka, is a dedicated botany student on the verge of pursuing her Ph.D., I come to you as a passionate aquascaping enthusiast without any formal background in the subjects discussed here. Our blog content is shaped by firsthand observations at Ripples Nature Aquarium Gallery in Kolkata (, India, coupled with extensive internet research conducted by Anushka and a little help from ChatGPT for me. While we strive for accuracy, inadvertent errors might have occurred with our conclusions and findings, and we welcome feedback from our valued audience to enhance our content.

Algae, a diverse group of organisms integral to ecosystems, have existed for over 400 million years, showcasing remarkable resilience in diverse environments such as saltwater, marine and freshwater, wastewater, and even on moist soil or rocks. These unicellular or multicellular photosynthetic organisms harness light energy through pigments, converting it into chemical energy much like plants, and contributing significantly to the world's oxygen supply. It's noteworthy that algae are responsible for producing over 70% of the Earth's oxygen and are extremely critical for our as well as the survival of all living animals.

Algae, with its minimal growth requirements, thrives in environments where many organisms struggle to survive, fostering life in diverse ecosystems. In our aquariums, equipped with essential elements like water, carbon dioxide, light, and suitable temperature, algae growth is inevitable. Viewing algae not as a nuisance but as an indicator becomes crucial — their presence or absence sensitively reflects the health of the ecosystem. The absence of algae in a well-equipped aquarium is cause for concern, signaling a potential imbalance.

But at the same time, overgrowth of algae robs an aquascape of its aesthetic appeal, and hence controlling algae and maintaining a balanced aquascape remains a ubiquitous challenge for all of us aquascapists.

While there are many resources online discussing why algae grow in planted aquariums, not many delve into the connection between water chemistry and algae growth. A specific observation at Ripples Aquascape Gallery in Kolkata, India ( sparked my curiosity, prompting a deeper exploration into the impact of water parameters on algae growth, with a particular focus on pH and alkalinity.

The 60cm (2ft) tank shown below was set up at Ripples around September or October 2023. As you can see from the picture below the tank suffered from a severe algae outbreak for quite some time. Plant growth too was not satisfactory.

But another 90cm (3ft) aquascape set during the same time as shown below was doing perfectly.

In terms of soil, fertilizer, water, temperature, CO2 injection, and light, both tanks had nearly identical parameters. The only difference was in the hardscape used. Tank A used rocks that raised the alkalinity while the rocks used in Tank B were of pure volcanic origin which didn't affect the alkalinity or pH of the water.

So we started studying how the pH and alkalinity of water affect algae as well as aquatic plant growth. Before proceeding further let us quickly refresh our knowledge of pH and alkalinity of water.

The pH of water, akin to temperature, is a vital characteristic. It is a value indicating the acidity or alkalinity on a logarithmic scale. The scale, ranging from 1 to 14, designates pH 7 as neutral, with lower values denoting acidity and higher values indicating alkalinity. The pH level significantly influences the ionic state of a solution, impacting living organisms and altering the solubility of chemicals such as heavy metals and nutrients. Both high and low pH levels can adversely affect the ecosystem by changing the availability of essential nutrients for plants and affecting the solubility of chemicals in the water.

Aquatic plants are known to thrive in the pH range of 6.5 to 7.5, but the majority of them prefer an acidic pH range of 6.5 to 6.8.

Algae on the other hand can survive in a wide pH range from 6.5 to 9.0. In general, the value of pH in the range from 7.0 to 9.0 is suit for most algae growth (Shapiro, 1984, Van der Westhuizen and Eloff, 1983, Williams and Turpin, 1987). Diatoms (a.k.a Brown Algae) grow best at a pH of around 8.1 and their growth rate decreases at a pH of around 8.5 (Morten Taraldsvik & Sverre Myklestad (2000)).

Both plants and algae need a primary macro (N, K, and P), secondary macro (Calcium, Magnesium), and Micro Nutrients (zinc, manganese, aluminum, and iron) to survive. Let's now look at the following chart which explains how pH affects the absorption of nutrients by plants. Please note that while this chart is for terrestrial plants, we are assuming that aquatic plants will also behave similarly.

It can be inferred from the chart that most nutrients are optimally available for the plants in the pH range of 6.2 to 7.3. Both highly acidic and highly alkaline environments are not suitable for the plants. So if your aquarium water pH is not in the optimum acidic range then plants will not be able to absorb all necessary nutrients effectively and they will become weak progressively. Alkaline conditions can also influence the interactions between different nutrients. For example, high levels of calcium in an alkaline environment can interfere with the uptake of other essential nutrients like magnesium. A very important micronutrient like Iron becomes less soluble and hence not available for uptake by plants in alkaline conditions i.e. pH > 7.

We have already seen that algae can survive in a wide pH range and can absorb nutrients in a wide range of pH conditions, including alkaline environments. The ability of algae to absorb nutrients in alkaline conditions is attributed to their simple structure and adaptation to various aquatic environments. Let's understand the adaptations that help algae absorb nutrients in diverse pH and alkalinity conditions in a little more detail.

  1. Large surface area compared to their cell volumes allows algae with more exposure to the surrounding water, allowing for increased contact with dissolved nutrients and hence more uptake of nutrients.

  2. Thinner cell walls reduce the barrier for nutrient movement into the algae cells.

  3. Direct absorption enabled by simpler cell structure assists Algae to absorb nutrients directly from their aquatic environment. Plants on the other hand need more complex root structures (and maybe leaf structures for aquatic plants) for nutrient uptake

  4. Efficient nutrient uptake thanks to specific proteins embedded in their cell membranes to facilitate the transport of ions and molecules.

If you have come this far then we are sure that you have understood that algae will not find it too difficult to absorb nutrients in alkaline conditions where most aquatic plants are likely to struggle.

With that established let's circle back to our Tank A and B and see what most likely was happening there.

Water in Tank A because of the higher content of calcium content in the rocks was becoming alkaline over time. Thus plants were finding it difficult to absorb nutrients. So nutrients were becoming more available for algae and with favorable light, CO2, and temperature conditions algae were proliferating. You can check the photo above once again and in fact, see a lot of brown diatoms which if you remember do very well in higher alkaline conditions (pH 8.1).

But in Tank B the rocks were not affecting the water parameters at all and hence conditions were more favorable for plants to absorb all available nutrients and out-compete the algae.

Before we summarize and come to the end of this blog, let's quickly look at the condition of Tank at the time of publishing the blog (February 29th, 2024).

The algae has reduced significantly and we can see more plant growth. We guess one of the reasons behind this is because available calcium on the rock surfaces has reduced and water is not getting alkaline thus making it favorable for the plants to out-compete the algae for nutrients. Needless to say, CO2 absorption has also improved making it easier for plants to photosynthesize better. You can check this article ( in this context.


  1. Alkaline water affects nutrient uptake by plants but not that much for algae. Hence you must check your supply water parameter

  2. Rocks used as hardscape can change water chemistry making water alkaline thereby favoring algae growth. Do check the rocks and sand and even the filter media that you plan to use in your planted aquarium

  3. If you cannot avoid alkaline water conditions then try to go for plants like Valisneria which thrives well in alkaline water conditions


I take this opportunity to introduce my co-author of this article, Ms. Anushka Dutta. She is a botany student and shortly will begin her Ph.D. in Plant science at Pennsylvania State University.

She enjoys teaching, communicating, and researching about nature specifically plant life.

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