How to Lower the Ph in Aquarium (Easy Guide)

Fish in the aquarium

Even though fish live underwater, they need plenty of oxygen to live long, healthy lives in your personal aquarium. Without that oxygen, fish will suffer just as a human would if they were living in an oxygen-starved environment.

If your fish appear to have low energy, seem sick or lethargic or simply do not appear to be thriving, you may need to check the pH in your aquarium.

It could be too high which creates an oxygen-poor environment for your scaly friends. In that case, you will need to learn how to lower pH in aquarium so that you can revive your pets before it is too late.

What Is PH?

The term pH is used to measure the relatively acidity or alkalinity of any substance, including water. You likely remember the term from a high-school or college chemistry class.

It probably never came up in the context of how to lower pH in aquarium but rather in terms of measuring how acidic something was and then changing the liquid in question to a more alkaline, or basic, measure.

We can generally classify pH in three ways:

  • Basic, or alkaline, which means that the substance in question would be receptive to hydrogen ions (more on this in a minute)
  • Acidic, which would mean the substance in question is more likely to “give away” hydrogen ions 
  • Neutral, which means the substance in question is right in the middle and is unlikely to react by either giving away or taking hydrogen ions

The amount of hydrogen in water is directly related to the amount of oxygen in water. This affects how well your fish can breathe.

More free hydrogen ions lead to more highly oxygenated water, which is why although most fish like pH measurements that are relatively neutral; they tend to do a bit better in mildly acidic environments over alkaline ones.

What Numbers Define PH?

What Numbers Define PH

Now that we have conducted a mini-refresher course on basic chemistry, let’s go ahead and start applying these concepts to how to lower pH in aquarium. A low pH measurement indicates that a substance is acidic.

This means it has a lot more hydrogen than something that matters a lot to your fish:  pure water. Battery acid, for example, has a pH of less than 1.

Orange juice, which is not a good home for your fish, has a pH of about 3. Milk has a pH of about 6 and pure water, which is considered pH neutral, has a pH measure of about 7. Sea water has a pH of about 8.

On the other end of the spectrum, soapy water, which is not good for fish, has a pH of about 12, and liquid drain cleaners are highly alkaline with a pH of 14, the highest measure possible.

Incidentally, your fish will thrive in an aquatic environment where the water has a pH measurement between 5.5 and 7.5 in nearly all cases.

The Right Way to Measure PH

There are many ways to measure pH. Measuring the pH in your aquarium is the first step in learning how to lower pH in aquarium because you will not know if you have a pH problem until you can properly evaluate the pH of your fish’s water.

You can purchase pH testing kits to help with these measurements, or there are many instruments that will give you a digital readout of the pH if you are not comfortable using the testing paper and a color gradient to determine the aquarium’s pH levels.

You have likely seen lifeguards at the swimming pool measuring pH levels to make sure that the water is not too acidic or alkaline for humans to swim in. You are basically going to be doing the same thing for your fish.

Why Is Correct PH Important?

A lot of aquarium hobbyists refer to the pH measurements of their aquariums as “normal.” However, the term “normal” is not appropriate when describing pH because a pH measurement that is good for one species may be terrible for another.

For example, certain types of bacteria called acidophiles actually love highly acidic environments that would kill other creatures. They thrive in pools of sulfuric acid.

If you happened to be cultivating acidophiles, then a “normal” pH for your aquarium might be somewhere between 1 and 5. 

On the other hand, certain types of trout have adapted over time to live in extremely alkaline environments. Their “normal” pH measure might be nearly 10, which would be far too high for most aquarium fish.

The Correct PH Matters for Every Aquatic Organism

Correct PH Matters for Every Aquatic Organism

While “normal” pH is hard to define, getting the pH correct is vitally important to keeping your fish alive and healthy. In most cases, aquarium-dwelling fish do best in waters with pH measurements between 5.5 and 7.5.

A saltwater fish will likely prefer a slightly higher pH because saltwater is more alkaline than pure water, registering around 7.5 or 8.

When dealing with aquarium pH levels, it is important to remember you can go too low as well as too high. There is a delicate balance in play that revolves heavily around what type of fish you have in your aquarium.

Acidity and alkalinity will both affect fish in the extreme, but every fish has a different extreme. For example, small mouth bass begin to grow more slowly and compete less effectively for food when pH falls below 6. Brook trout compete for food just fine and tend to be quite large at pH levels as low as 5.

When fish cannot compete for food or are lethargic and slow in the wild, they experience immediate, negative effects and in many cases, are eaten by something feeling less lethargic.

While a fish may not necessarily be in danger of consumption in your home aquarium, high pH levels or extremely low ones can prevent your fish from getting enough to eat and thriving in the environment in which you have placed them. 

How to Lower PH in Aquarium Safely:

How to Lower PH in Aquarium Safely

You must know how to lower pH in aquarium safely, or you may end up with a tank full of water with the wrong pH and, sadly, a bunch of dead fish.

Going back to the swimming pool example:  you probably have seen a lifeguard measure the pH in a pool, then add one or more scoops of powdered chemicals to the water to adjust that measurement. The results likely worked quickly.

Sometimes, lifeguards will even do something called “shocking” a pool to adjust the measurements even more quickly. In these cases, they may warn swimmers to stay out of the water if they have sensitive skin.

You may notice even after you have been cleared to get back into the pool that your eyes burn more if you open them underwater than they did before.

This is because the chemicals are still in the water lowering the pH and, in conjunction, irritating your eyes. Imagine, then, if you had to breathe through that type of pain instead of simply wiping your eyes, putting some drops in them, and not opening them underwater in the pool again.

A fish’s gills are sensitive just like your eyes are. However, unlike you and your eyes, a fish cannot remove their gills from the water that is hurting them without suffocating and suffering a horrible death.

The only solution for a poor pH environment for your fish is to learn how to lower PH in aquarium safely.

Using Natural Methods to Lower PH

Using Natural Methods to Lower PH

In the natural world, fish live in water with pH that is, in most cases, suited to their species’ needs. This is why some fish live in the ocean while others live in swamps.

If you need to adjust your aquarium’s pH levels by lowering them naturally, consider trying:

  • Peat moss
  • Driftwood 
  • Cappata leaves

Peat moss is a popular option for those learning how to safely lower pH in aquarium because it is very natural and commonly grows in areas where many aquarium fish naturally live.

However, peat moss may give your aquarium water a brownish tinge. You can alleviate this effect by soaking it for a few days in a bucket before adding it to your aquarium.

You can also buy peat pellets or peat chunks to place in your aquarium. You must leave the peat moss in the aquarium for it to lower the pH, so there will likely be some discoloration in the water over time.

Driftwood comes with the same benefits as peat moss when it comes to lowering pH naturally, but it also may discolor the water in your tank. Furthermore, driftwood is not all the same, so you will need to do some research to make sure it will not:

  • Disrupt your tank
  • Release problematic chemicals into the water 
  • Create a dirty environment for your fish

Avoid these issues by boiling driftwood in saltwater to sterilize it and remove dirt and debris, and make sure any driftwood you purchase for your aquarium is clearly labeled as safe for and intended to be used in fish environments.

Some driftwood is sold specifically for reptile environments and has very different characteristics that can harm fish.

Catappa leaves, which are also known as “Indian Almond leaves,” lower pH and may have antibacterial qualities that could help keep your fish healthy.

As such, they may offer the best option for how to lower pH in aquarium naturally. These leaves may even cure fin rot and protect fish babies (fry) from various infections.

Like peat moss and driftwood, catappa leaves will discolor the water in your tank. Some aquarium owners soak the leaves until they decompose, then add the stained water to the tank when they change the rest of the water in order to combat this effect. Unfortunately, there is  no escaping the discoloration entirely.

Using a Reverse Osmosis Filter

Using a Reverse Osmosis Filter

If you do not think you can stand the look of discolored water in your fish tank, then a reverse osmosis filter may be the solution for you when it comes to how to lower pH in aquarium to keep your fish happy and healthy.

Reverse osmosis, abbreviated RO, is a filter system that removes contaminants from the water while still allowing smaller ions to pass through the filtering system.

The result is a lower pH, since most larger contaminants are those that contribute to higher pH levels.

There are a few negatives to using an RO system, however:

  • Cost (RO systems are expensive.) 
  • Bulky and will not work for most small aquariums 
  • Filter change (You will periodically have to replace the filter.)

On the positive side, if you use an RO filter, you can be confident it is removing:

  • Pesticides 
  • Arsenic 
  • Heavy metals

All of those contaminants are terrible for your fish and contribute to shorter, unhealthy lives for your aquatic friends.  


The pH in your aquarium is of the utmost importance to your fish. Professional aquarium caretakers and owners emphasize that more than just understanding how to lower pH in aquarium safely, you must understand when to start trying to adjust pH.

While we can tell you, correctly, that most fish like pH measures between 5.5 and 7.5, another aquarium expert may tell you the magic window lies between 6 and 8.

Still another will tell you that none of this matters at all as long as your fish are healthy, lively and living relatively long lives.


Chasing the “perfect pH” by constantly trying to better learn how to lower pH in aquarium can create a terrible situation for you and your fish in which you are constantly interfering with their environment and they are constantly trying to deal with changes in their miniature ecosystem.

Do not inflict your desire to micromanage your fish tank on your pets. Instead, monitor their health, and respond to their behavior only when necessary.

The result will be an aquarium with happy, healthy fish living in an environment with a pH that is “perfectly normal” for them.


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