How do Fish React to Low pH in an Aquarium?

Those of us who love to keep fish in aquariums know that we have to switch out the water pretty frequently.

To assist in that, aquarium filters were invented. However, who has seen watched Finding Nemo knows that those filters can still break and the aquarium can get dirty.

There’s a lot more to taking care of fish in the aquarium than changing the water regularly. There are other things, like the pH, that you have to account for.

Not only is a balanced pH level vital to the lives of the fish, but their development is also dependent on it.

A difference in pH can make a pretty big difference in the health and growth of the fish.

Controlling that pH to be just right is essential for the survival and health of your pet fish.

What you may not have known is that there are natural processes that can lower the pH of your aquarium.

These processes are all around us in nature. In fact, many processes taking place inside your own digestive systems are acidic!

That is why you have to brush your teeth with alkaline toothpaste to balance out the acidity in your mouth. It’s why there are tablets for acidity and digestion.

Here’s how it all connects with your aquarium and how you can contribute to keeping your darling fish healthy.

What You need to Know about pH and Aquariums?

Perhaps you remember studying pH for a brief moment in science class during the 7th or 8th grade.

It’s a measure of the acidity or alkalinity of a substance; mainly of water or a solution. The higher the pH, the more alkaline the solution, the lower it is, the more acidic it is.

However, what does that mean in its essence?

pH is measured on a scale of 0-14, 0 being the most acidic, and 14 being the most alkaline.

The variance of this scale depends on the concentration of hydrogen ions (H+) in the solution.

Hence, the scale is based on the power of Hydrogen (pH) in a solution.

There is another ion that contributes to the scale, the hydroxide ion (OH-). It’s a combination of hydrogen and oxygen ion. The two combine to make water.

The more hydroxide ions there are in a solution, the lower the concentration of hydrogen ions in comparison. That allows the pH to rise and become alkaline.

High or low pH isn’t necessarily bad for everything. For instance, detergents and washing soaps are usually all alkalines. Car batteries are acidic.

Certain foods that you eat like lemons and oranges are acidic while limes are alkaline.

However, there has to be a strict check on the environment that certain animals live in.

If that environment changes to become acidic or alkaline, it’s detrimental to their health. Since fish live in water, they’re more susceptible to this change than other creatures.

What is Normal pH?

Normal pH is that of distilled water.

It basically means that there is an equal amount of hydrogen and hydroxide ions in the solution.

This basically makes the solution water. This is the pH that most life survives at.

It allows most animals, including fish, to thrive. On the pH scale, this is 7.0.

The Importance of pH in Aquariums

Many fish actually thrive at and around normal pH.

This is why you’re supposed to switch out the water in the fishbowls and why aquariums have filters. Angelfish, Goldfish, Clownfish, the Zebra Danio, and the Neon Tetra, all thrive at or around normal pH.

While they have certain variable parameters around which they can survive, deviations can lead to problems.

However, there are other fish that thrive in narrow pH environments. For example, there are fish like Discus.

These don’t like even slight variations of pH and require great care.

You must also know that sudden changes in the pH of an environment are never helpful. This is why filters are preferable to switching out the fishbowl water each night.

Animals, including fish, respond well to slight changes rather than incredibly forced, rapid changes in their lifestyle.

Biggest Cause of Low pH in Aquariums

Fish Illness or Death is a pretty common cause of low pH in aquariums. Now, you’re not supposed to call in a vet every time this happens, of course.

But you should take it upon yourself to find out the signs of fish illness.

You should also consult with the vet as to which medications are best. This way, you will know to test for any pH changes.

This should carry on through the medication of the fish in the tank.

The daily checks should only end when the fish has been cured and the medication has stopped. Remember that checking the pH should be strictly on schedule.

The pH of the aquarium is liable to shift throughout the day. Taking it at different times of the day every day may prove counterproductive and fuel unnecessary fears.

Effects of pH change on Aquarium Life

A lot of freshwater lakes and rivers have a pH of 6-8. This includes streams and ponds as well.

It’s also the standard pH of an aquarium. However, unlike any of the freshwater bodies, an aquarium doesn’t have a constant flux of water.

There is no great water cycle providing nutrients and fresh water to it. This is why you need to control the pH yourself.

You need to understand that the more the pH deviates from the norm, the higher the risk is.

It’s nearly 10 times more dangerous for fish at 4.5 than it is at 5.0. The same goes for when the pH rises above 7.0.

Low pH in Fish Tank

The effects of low pH on aquatic life can be devastating. For example, just as how acid rain can kill fish as the pH falls below 5, so can neglecting your aquarium.

When the pH approaches 5, not only are the fish’s vital processes affected, but plankton and mosses may begin to thrive.

This leads to a power struggle that can lead to the fish’s death.

Below that, around 4.5, there is a very slim chance of survival for most fish. At that point, it’s mostly game over.

High pH in Fish Tank

Very high pH is also detrimental to fish. It can kill adult fish directly as well as other invertebrates.

For juvenile fish, it can damage their development and render them essentially stunted or disabled.

The slime coatings that fish have will develop chaps in the skin due to the alkalinity.

Other damages can include erosions of the skin in various places. The eyes, the gills, and any exposed surface will be under attack.

The Aquarium’s Nitrogen Cycle

In freshwater systems, the pH determines whether the waste of decomposing fish food will become ammonium or ammonia.

The former means that the water will become acidic, and the latter means it will become alkaline.

As the pH dips below 6, the bacteria that digest ammonia start processing it in water.

If they rise above 9, toxic aqueous ammonia may be generated. Both these occurrences will eventually kill the fish.

Also read: How to Start Nitrogen Cycle in an Aquarium

Testing pH in Aquariums

There are several ways to test the pH in your aquarium.

Digital Tester

You can go the easy route and just purchase a digital tester for faster reading. You should ideally test your tank once a day or twice a week if you’re not too fussy.

You should also test it when there are any major changes made to the tank like the introduction of a new fish.

Remember to zero out or calibrate the tester regularly. Remember to follow the exact instructions. If you don’t pay attention to calibration, you may mess up the reading.

This is the more expensive option though. If you’re comfortable with a more old-school approach and don’t have the budget, go for chemical testing.

Also read: What Should Be the Water Test Results of My Aquarium?

Chemical Testing

This requires that you go through an entire process.

You need to purchase a whole kit including a glass tube and color card.

While this approach is more cost-effective, it takes time. You’re better off purchasing a digital tester.

Altering pH in Aquariums

It’s usually recommended that your fish adjust to the water in your aquarium rather than the reverse.

However, some fish do require that you set strict parameters and keep the pH in constant check.

Hence, you will have to either raise or lower the pH eventually.

Here are a few ways to raise or lower the pH in your Aquarium.

How to Raise pH in an Aquarium?

Below are some ways to increase the pH in your aquarium:

Regular Water Changes 

Regularly changing the water of your tank will remove the acidic water.

It’s better to get a filter for your aquarium rather than doing a complete water change daily.

This shocks the fish every time they are transferred to a new environment.


Adding crushed coral or rock salt will also raise the pH of your aquarium.


Increasing oxygen in your aquarium will also drive down CO2 concentration.

This will raise the pH of the water.

Baking Soda 

Baking soda raises the pH of your aquarium; however, a constant supply is needed.

Measurement is also needed since it’s basically a chemical that can largely change the environment.

Dissolve the powder in some water before the addition.

A great rule of thumb for adding baking soda is 1 tsp/ 5 gallons of water. However, you should delve more deeply into the literature if your fish require greater care.


Adding simple seashells to the aquarium will raise the pH.

This is due to their carbonate salt makeup.

Chemical Addition

Commercial chemicals are available to raise the pH in most general stores.

However, these rapidly change the pH of your water.

Careful measurement is needed for effective addition. These chemical buffers are only to be used when other options are exhausted.

How to Lower pH in an Aquarium?

Below are some ways to lower the pH in an aquarium:

Filtering through Peat Moss 

This is a pretty effective way to lower pH.

Some people also use peat moss substrate to induce the same thing.

Addition of Carbon Dioxide 

Increasing the CO2 levels in your water tank will lower the pH quite effectively.

However, a constant supply is needed if you are to keep it that way. Make sure that you measure the concentration of the gas flow rate needed. Otherwise, you may poison the fish.

Addition of Wood 

Driftwood is very capable of lowering pH. Adding some will get the job done.

Make sure you clean the driftwood (you can also boil it). This will ensure that it doesn’t add any thing harmful for your fish

Chemical Addition

This doesn’t maintain a stable pH since chemical addition is more suited for rapid changes.

There are various products available in the market to lower the pH, however.

Preventing Low pH in Aquariums

Here are a few steps you can follow to prevent the lowering of pH in aquariums.

Regular Checks

If you’ve heard that a watched pot never boils; you may have also heard that a watched aquarium never grows acidic.

If you keep regular checks on the fish that you’re keeping, they should be fine.

It all depends on how vigilant and careful you are. If you establish a routine around feeding them and checking the filters, you’ll be fine.

Check if you’ve recently added another fish to your aquarium and the aquarium has grown more or less acidic.

If you want to get scientific about it, you can keep a pH logbook.

This way you can track the time and date of any sudden changes. This will make it easier to pinpoint exactly what caused a sudden shift in pH.

Introduction Checks

When a new fish is added to the aquarium, you should check the water before you introduce it.

When you’re buying the fish, make sure to obtain these details from the shop.

The new fish may thrive in pH levels that are too narrow or removed from those of your old fish.

This may lead to the death of one or of your pets.

If you follow these procedures, you’ll keep your fish safe and the aquarium at just the right pH.

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