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How to Safely Lower pH in Your Aquarium

When setting up a new aquarium, testing the pH is something we all should be doing before adding fish. However, once we have fish, many aquarium keepers don’t bother keeping track.

Why, when so many captive-bred fish are resilient, tolerant of a wide range of pH levels, and tap water, though slightly alkaline, is usually fine for anything you’ll buy in a pet store?

What is pH in Aquariums?

From the Oxford Dictionary: 

  • pH
  • /ˌpēˈāCH/
  • noun: pH; plural noun: pHs
    • a figure expressing the acidity or alkalinity of a solution on a logarithmic scale on which 7 is neutral, lower values are more acid and higher values more alkaline. The pH is equal to −log10 c, where c is the hydrogen ion concentration in moles per liter.

This sounds complicated but it can be digested simply as a measurement of the concentration of hydrogen ions (H+) relative to hydroxide ions(OH-) in an aqueous solution. There’s more to it but this is all you really need to know, along with the fact that it’s not a linear measurement.

pH is a logarithmic scale; this means each unit is greater than the last one by a factor of 10.

pH 5 is an increase of 10 over pH6 in terms of acidity. pH9 is an increase of 10 in alkalinity over pH8. pH7 is a neutral balance of H+ and OH-.

This means that what might appear to be a small change in pH using a test kit is actually a significant change in chemistry that can have drastic effects on your tank ecosystem.

When is it Necessary to Lower the pH in Aquariums?

A green beautiful planted tropical freshwater aquarium with fish pterophyllum scalare

Usually we want to raise or lower the pH in aquariums for the health of living organisms. Many fish and plants come from regions of the world with stable, narrow water quality parameters.

If you keep tetras, barbs, angelfish, Amazon Swordplants, and other organisms from tropical, equatorial regions like Brazil or Southeast Asia, many of these creatures thrive in acidic waters. 

The streams, rivers, and lakes these fish and plants hail from usually flow through forests that drop leaves, sticks, and driftwood rich in tannins. The soil is also rich in tannins, humus, and other organic materials.

And as it turns out, tannins and other organic matter has the tendency to push water pH towards acidity.

While many captive-bred aquarium fish aren’t so picky, you’ll often get better color and breeding behavior when you match their pH requirements.

African cichlids from the rocky, mineral-rich rift lakes and reef fish thrive in alkaline waters. Lime-rich coral and rocks that leach minerals cancel out acidity and raise the pH of water beyond 7.

Fortunately, since tap water tends to have dissolved minerals in it, it tends to be easier to keep aquariums slightly alkaline vs acidic. But how can you lower the pH in aquariums instead?

Tools to Monitor the pH in Your Aquarium

Aquarium pH strip over comparison chart

The first step in know whether or not the pH in your aquarium needs to be adjusted is getting an accurate reading. Here are a few recommended tools to monitor the pH on your tank.

pH Water Test Kits

pH Reagent-based drip kits like API’s Pond Testing are the classic way to test your pH. By adding water into a tube and adding the test reagent, you can watch as the water changes color.

Match your water’s color to an included chart and you’ll know with relative precision how acidic or basic your water is. These test kits are inexpensive, easy to use, and offer a fair amount of accuracy.

Dip test strips are the second more common way to monitor pH.

They have the major advantage of being far more convenient than drip reagent test kits. You can simply dip your strip into your aquarium and compare the results to a handy color table within 30 seconds to a minute.

Dip test strips like Capetsma’s 9 in 1 Aquarium Test Strips can even simultaneously test pH, kH, nitrate, nitrite, ammonia, iron, and other useful measurements all in a single swipe. I prefer these for general water quality monitoring because they’re fast and accurate enough.

But when making fine adjustments to lower the pH in aquariums, I don’t like relying on the vagaries of color comparison.

Digital meters give me pH results down to 0.01 rather than .1-.5 using color charts.

This Vivosun model is my favorite tool when performing water changes and any sort of chemistry adjustments because I can know the specific pH and temperature in seconds. 

How to Lower pH in Aquariums – Recommended Methods

Here are a few recommended methods to lower the pH in your aquarium:

1. Chemical Solutions

Possible the most common way to lower the pH in aquariums is using a bottle of chemical solution.

Several manufacturers like Fritz and Kordon offer products, all with labels like “Water Softener,” “pH Reducer,” “pH Lower,” and so on.

The active ingredients vary, some using diluted acids and others using “all natural” methods like tannins and other organic matter that has an acidic quality.

All should be used specifically according to the directions on the label, with special care taken to avoid skin and mucous membrane contact.

2. Driftwood

Of all the tools available to lower the pH in aquariums, driftwood is my favorite.

It looks stunning, provides cover for fish and invertebrates, attachment points for aquatic plants, and helps create acidic, tea-colored water for a rainforest river-style aquarium.

Dense, tannin-rich woods like Colophospermum mopane (Mopani wood) are ideal.

It not only sinks easily but also acts as an acidic buffer thanks to the continual slow release of tannins plus the natural microbial decay of wood over time.

Lighter woods like cholla cacti tend to have a minimal effect on pH. While their decay will create free-floating organics that reduce pH they aren’t nearly as helpful as denser tree-based driftwood.

If your driftwood floats when first purchased, filling a bucket with water and weighing down the driftwood for sometime between 24 hours and a week will allow water to drive out residual air.

This will allow your driftwood to sink naturally and stay at the bottom of your aquarium. If the wood is of neutral buoyancy, you may need to weigh it down with rocks, silicone it to the tank glass, tie it to more stable decorations, or find other creative solutions to keep it in place.

Many people suggest boiling driftwood to sterilize it.

If you’re purchasing it at a pet store, I wouldn’t bother as it’s usually already treated for aquarium use and prolonged boiling will also drive out the very tannins we want to keep the pH low.

Pieces found in nature should be briefly boiled to kill off undesirable organisms, however. 

Or if you live in a warm region, you can let new driftwood sun dry for a few weeks to kill harmful multicellular organisms like worms, snails, insects, eggs and parasites.

Any driftwood collected in nature should have the bark removed beforehand as bark can collect water and keep these creatures alive long enough to make it into your aquarium.

3. Peat Moss

Another popular tool for lowering pH and keeping it there is peat moss. By weight, peat moss is even higher in tannins than driftwood.

It also has the dubious side effect of turning your water the color of tea if you use enough of it. If you want this more natural look then its a definite plus but if you prefer crystalline water, peat moss may not be ideal.

I prefer using peat moss as a substrate additive used while setting up new aquariums.

It’s a common ingredient when mixing substrates for planted aquariums as it buffers the pH towards acidity and slowly releases organic matter directly to plant roots as it decays.

Also, adding peat moss before adding fish or plants I can get my water chemistry under control without hazarding their health.

Alternatively, you can use a small, water-pervious container or even a single pantyhose with a small ball of peat tied into a knot. Place it in a section of the aquarium with good water flow to ensure tannins get released without peat freely floating about.

Adding your bag of peat to a hanging filter or canister filter will cause your water pH to slowly decrease over the course of a few days. 

The challenge is knowing precisely how much and how quickly your pH will drop because peat varies in acidity depending on its age and other factors.

The size of your tank and its buffering capacity and other chemistry factors will all affect how well your peat moss works.

Personally, I’d never use the peat method on an aquarium under 10 gallons. There’s too little water and the tank pH can shift lethally within hours. 

4. Catappa Leaves

Indian Almond (Terminalia catappa ) leaves are increasingly common in pet stores today. I prefer these to using peat in established aquariums.

Like both driftwood and peat, these leaves slow-release tannins through their decay. While turning your water brown, they help decrease and buffer the pH towards acidity.

I find Indian Almond leaves to be a bit more precise in terms of measurement; roughly a leaf per 10 gallons is enough to cause a gentle yet noticeable reduction in acidity.

I love the natural look and prefer to let place them in choice locations around the tank.

They will eventually wander, getting caught in currents, and providing natural sailboats for small shrimp and snails.

You may want to instead bundle them and place them within the filter, where they will continue to add tannin without looking unsightly.

5. CO2 Reactors

While not usually used specifically for acidity, if you use live plants, you really should be looking at using a CO2 reactor.

These units provide a constant stream of carbon dioxide, a gas critical for plant growth. CO2 is also mildly acidic and will not only help lower the pH in aquariums but keep it there so long as the reactor is running.

Plants in CO2 rich aquariums often produce so much O2 it forms streams of bubbles on the leaves as they busily photosynthesize, giving you a visual of the gas exchange cycle of your ecosystem!

6. Water Changes

If your aquarium pH is already high (greater than pH7), combining your efforts to reduce pH with a water change is a good idea.

Dissolved minerals need to be cancelled out before you actually see a reduction in pH. And pure water is very close to neutral, with tap water ranging from neutral to slightly alkaline.

Aquariums that simply get topped off with fresh water rather than water changes inevitably become alkaline over time. When water evaporates, dissolved minerals get left behind.

Because tap water is usually slightly alkaline, you have a net increase in dissolved minerals like carbonate if you don’t perform water changes.

Once you’ve flushed out some of the alkaline-buffering agents, you’ll have an easier time lowering the pH of your aquarium.

Remember that this means your pH will also drop faster; give your fish time to adjust to the initial pH change that comes from the water change itself before reducing it further via other means!

7. Reverse Osmosis Units

If your municipality has particularly mineral-rich water, you may want to invest in a reverse osmosis (RO) unit.

Much like aquariums, tap water full of dissolved minerals can cause problems with lime scale buildup and other problems over time.

Many homeowners also use RO units where the groundwater contains sulfur and other noxious agents. Reverse osmosis work by forcing water through semi-permeable membranes that trap heavier ions and allow light molecules like water to flow through them.

The main downsides of using a reverse osmosis unit are the price and maintenance involved. RO units are never cheap to install and require regular maintenance, making them impractical to use purely for aquarium water.

However, if you already intend on installing one as a homeowner, they are useful as a source of aquarium water that’s far closer to neutral pH.

One Final Tip on Lowering the pH

One thing that can’t be emphasized enough: do not drastically reduce the pH all at once.

You need to be particularly careful when making adjustments in smaller fish tanks because it won’t take much of an additive to make a change.

Remember how pH6 is 10 times more acidic greater than pH7?

Because pH is a logarithmic measurement pH5 is 100 times more acidic than pH7. An extra capful of conditioner can devastate a 5 gallon nano-aquarium if you mismeasure.

Fish and plants vary tremendously in how well they tolerate adjustments to pH so I always err on the side of caution.

If I’m using chemical means, I want to adjust 10-20% of the water volume at a time per day until the pH is where I want it. Most of the plant-based organic methods will cause a similar adjustment over time if done right.


Knowing how to lower the pH in aquariums is one thing but there are several ways of going about it.

Each has pros and cons you’ll want to consider; if you’re setting up a tank for the first time, you have more flexibility and safety in terms of getting things right.

If you’re making adjustments with fish and plants already within the ecosystem…While it’s still easy, just be careful to follow any directions that come with your chemical or additive of choice and regularly test the water to ensure your parameters don’t get out of hand. Best of luck with your aquarium!

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

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