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How to Safely Raise the pH in Your Aquarium

Many community fish keepers may not bother much with the pH. After all, their fish can thrive in a wide range of water conditions. But what if you’re keeping something a bit more specialized, that prefers exclusively hard, alkaline water conditions?

If you’re trying to figure out how to raise the pH then you’ve come to the right place! Let’s get started by discussing what exactly pH is and why it’s important to keep track of!

What is pH?

pH is a chemistry term that’s used across the branches of science when discussing the acidity or basicity of various substances, usually liquids. Since that probably came across as a bit technical, I’ll delve into this a bit further.

The pH of a substance is a measure of the concentration of hydrogen ions/protons (H+). A substance with a pH of less than 7.0 has a high concentration of these protons and they tend to react with substances more effectively.

Pure water has a pH of 7.0, meaning it’s perfectly balanced between acidity and basicity. While the water is far from pure in any aquarium, it’s still possible to balance the reactivity of the aquarium. A basic solution has a pH greater than 7.0. It will have higher concentrations of OH- ions that can counteract the H+ in solution.

Something that’s very important to remember about pH is that it’s a logarithmic scale. The formula is: pH=-log[H+], which you don’t need to know. But do know that each step of pH is a base 10 change in concentration.

Meaning pH 4 is 10 times more acidic than pH 5; and 100 times more acidic than pH 6! This is why it’s so important to measure pH; even minor shifts can have drastic effects on water chemistry and your aquarium inhabitants!

Most bodies of water are also highly stable in terms of pH. Occasionally you do see rapid shifts, especially in freshwater environments where floods, rains, forest fires, and other events can cause localized shifts in water chemistry.

But one reason saltwater fish are considered difficult is that they are used to a highly stable environment, especially reef fish and corals. Even small shifts in pH are huge compared to the changes that usually take place over thousands or millions of years in nature. They simply aren’t adapted for rapid changes in chemistry.

General Hardness and Carbonate Hardness

While pH is the measure of H+ in solution it’s also affected by GH (general hardness) and KH (carbonate hardness). These are measures of dissolved minerals in your water. General hardness is the concentration of magnesium and calcium (how “hard” your water is) and carbonate hardness measures carbonates and bicarbonate, which have a counteracting effect on H+.

KH is, simply put, your alkaline (pH >7.0) buffering agent. The greater your KH, the more resistant it is to swings in pH that would shift it towards acidity. Therefore, if you want acidic conditions for certain fish, lowering your KH allows plant tannins and other sources of H+ to bring the pH below 7.0.

A high GH goes hand in hand with a high KH because carbonates aren’t usually found free in nature. They come bonded to calcium and magnesium salts; therefore a high GH almost always signals a high KH. But in a solution with water, these ions are free to disassociate from their metal partners, potentially interacting with H+ in water to raise the pH.

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When is Maintaining a High pH Important?

Maintaining a high pH is not important for all fish. Many freshwater species come from neutral or even acidic conditions. Certain equatorial tropical fish, such as Discus, Tetras, and Loaches, all prefer soft, acidic water. This is due to the flooded tropical forests they live in. Plant tannins, decaying driftwood, organic-rich soils, and other debris all release chemicals that buffer the water towards acidity.

Also, the vast majority of tropical fish are bred in home aquariums or massive fish farms. And these places tend to use local water that’s typically alkaline in chemistry. Since these fish have adapted over the decades, you may not have to provide them with acidic conditions.

Still, if you try to do so, you may see better color, health, and greatly increase your chances of them spawning. Tetras and German Blue Rams, for instance, are next to impossible to spawn without neutral to acidic water conditions.

Other fish, such as African Cichlids, and Livebearers, prefer hard, alkaline water. While many of them will thrive in neutral or even acidic conditions (especially Guppies), you’re much more likely to keep them healthy in hard water! Like their soft water counterparts, Frontosas and other hard water fish are unlikely to spawn in neutral to acidic water conditions.

Freshwater and marine invertebrates almost all prefer high pH conditions as well because of their hard, mineralized shells and exoskeletons. These hard parts use dissolved minerals that are eaten away if the water conditions grow too acidic.

If you haven’t already, take the time to test the water coming out of the faucet with a pH liquid test kit or testing strips. Water from the faucet is typically hard and alkaline in most countries because the minerals have little impact on human health and don’t need removal.

At worst, they can build up over time, requiring costly repairs. But acidic water has its own problems, such as leaching away at metal pipes, and neutral water would be too expensive to provide for everyone.

How does the pH Change in an Aquarium?

pH isn’t a stable water parameter and it can change over time as different chemicals build up. Understanding what impacts the pH can prevent stress in your aquarium residents!

Organic Matter Build Up

Organic matter, especially plant debris, has a major influence on the pH of your tank. As I mentioned before, plant tannins from decaying matter tend to shift the pH towards acidity. While it’s not enough to drastically shift hard water into acidic water, plant tannins will reduce its buffering capacity (its ability to resist rapid pH swings).

Water Evaporation

One common way the pH changes in an aquarium is through evaporation. Any tank will slowly lose water over time as it evaporates. The drier your air and the warmer both the air and water are, the faster it will evaporate. Aquarium aeration, while important for oxygen and carbon dioxide exchange, also speeds up water evaporation.

The thing to remember is that while water leaves your tank and the water level drops, the dissolved substances it contains don’t go anywhere. This is how we end up with hard water stains in drinking glasses or on the sides of a tank. And as the water evaporates, the concentration of dissolved minerals increases, which tends to increase the pH of the tank.

When doing water changes, you may need to perform extra-large ones on occasion. Because if you only top off your tank with more mineral-rich water, the pH will steadily climb if you allow too much evaporation to occur between water changes. While hard water fish will not mind this effect, many fish that thrive in a moderate pH range, such as Bettas and Tetras, will start to suffer.

Water Changes

Performing large water changes can also rapidly shift the chemistry of your tank. If you have neutral to acidic water and you perform a water change using untreated basic tap water, you’ll shift the conditions towards basicity (alkaline). If you have a large concentration of plant tannins (acid buffers) or crushed coral substrate (alkaline buffer), these can help neutralize any changes to the water chemistry that your change would have, however!

Low KH (Alkalinity)

Remember what I said about KH before? Large amounts of carbonate help negate increases in H+, which would normally drop the pH towards acidity. But if you have a low KH then your pH can easily shift. Sources of H+ (acids) include decaying plant matter and carbon dioxide; planted tanks using injected CO2 are notorious for rapid pH swings for this reason.

How do we Raise and Maintain the pH of an Aquarium?

Raising the pH is fortunately, very simple and effective. However, one should always be careful not to do it too quickly. Remember that each shift in pH is a 10x concentration adjustment. Going from pH 7.0 to 8.0 can be lethal if done in a few hours or less. Here are some ways to (carefully) raise the pH of an aquarium and then keep it there!

Liquid pH Shifting Additives

The fastest way to raise the pH is to use liquid chemical additives. These are very effective but you should always follow the instructions to the letter because they cause instant shifts in water chemistry. Usually, you’ll only add a small amount, re-test your water, and then wait anywhere from 6-24 hours before adding more.

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The downsides of these chemicals is that without an alkaline KH buffer, your water may eventually shift back towards acidity. And like raising the pH, it crashing back to acidity is also stressful for aquatic life. So you should be monitoring KH as well to ensure your PH stays where you want it to be once you increase it.

If you know that you have soft or neutral water, you can also add liquid pH increaser to the water right before adding it to your tank. This way, you won’t see a sudden drop in pH during water changes!

Buffering Rocks and Substrates

Maintaining a stable, high pH is much healthier for your animals and plants long-term than constantly adding chemicals to raise it. And one of the easiest ways to maintain a high KH is to use a substrate that constantly releases minerals into the water to counteract acidity (buffering substrate).

Crushed coral (aragonite) is extremely popular among African Cichlid and marine aquarists for this reason. A substrate of crushed coral, combined with the right rocks, will keep your pH steadily around 7.8-8.5, depending on the other minerals in solution.

Rocks are another way to buffer the pH towards alkalinity. Seiryu stone, limestone, sandstone, shale, and other sedimentary and metamorphic rocks are the best choices for raising the pH because they all contain large amounts of calcium carbonate (pure GH & KH). Igneous rocks like granite and basalt tend to not have as much of an effect on the pH, which may be perfect if you’re keeping softwaterfish!

If you aren’t sure what kind of rocks you have and whether they will buffer raise the pH, you can test them by using ordinary white vinegar. Dry the surface you want to test thoroughly and then place a few drops on it and watch. If it begins to slowly foam, you’re seeing the acidity of the vinegar reacting with the carbonates in real-time, letting you know that this rock will buffer the pH!

Remember that not all rocks contain heavy amounts of GH and KH increasing minerals. You may not see the vinegar-based reaction but the rock may still contain some buffering capacity. So stick to rocks you know will shift the pH if raising the pH is important to you!

Boiling Driftwood

Since driftwood contains moderate to high amounts of plant tannins, it can act as an acid buffer for your water. Combined with a substrate rich in organics, like aquarium plant soil or peat, you may find it impossible to raise the pH in these tanks.

If you want to use driftwood but also want an alkaline pH, boil the driftwood for one to two hours beforehand. Make sure that you submerge as much of the mass as possible. Boiling will drive out a lot of the tannins, which are normally released very slowly over time.

You can also choose particular driftwood types that don’t contain much tannins to begin with, such as manzanita and spiderwood.

Removing Carbon Dioxide

Carbon dioxide is a plant nutrient and essential for photosynthesis. But it’s also a weak acid that will shift the pH towards acidity. In planted tanks it can be hard to counteract this effect but you can provide aeration in fish tanks to remove CO2 through surface agitation.

Remember, agitating the surface also speeds up evaporation, which increases the GH and KH through increasing the concentration of minerals.


Knowing how to raise the pH is a major part of an aquarist’s tool kit and one of the first steps you take in the journey towards expert fishkeeper! While raising the pH is important, maintaining it to where you want it to be is even more so. So never neglect buffering agents to prevent sudden pH swings from harming your fish or invertebrates!

Jason Roberts
About Jason Roberts
Jason is an aquarium fanatic that has been a fish hobbyist for almost three decades.

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