Symptoms of Low pH in Fish Tank

Photo of author

by Jason Matthews



For your fish to be happy, the water in their tank needs to have a pH level that’s as close as possible to neutral. If the pH dips too low or becomes too high, it compromises water quality and your pet’s health. If the pH is too low, the fish will show difficulty in breathing and an irritated gill structure.

So, what are the symptoms of low pH in a fish tank?

The growth of algae is a sign that the water’s alkalinity has dipped too low. Green water, sludge, spots, and brown streaks are all indications that your tank is suffering from low pH levels. If your fish have pale coloring, are laying on the bottom, or stay close to the surface, it could also indicate that they are experiencing difficulty breathing.

API pH UP Freshwater Aquarium Water pH Raising Solution 4-Ounce Bottle
2,832 Reviews
API pH UP Freshwater Aquarium Water pH Raising Solution 4-Ounce Bottle
  • Contains one (1) API pH UP Freshwater Aquarium Water pH Raising Solution 4-Ounce Bottle

Last update on 2023-06-05 / Affiliate links / Images from Amazon Product Advertising API

This article reviews some of the most common symptoms of low pH in fish tanks. Next, it explains how to test the fish tank’s pH level. Then, it describes how to adjust the fish tank’s water chemistry. Finally, you will learn how to monitor your fish tank’s pH level so you can keep an eye on it over time and keep up with its fluctuations.

Possible Symptoms of Low pH Level in Fish Tank

The following are some of the symptoms you can expect to see when your aquarium has a low pH level. This list outlines the symptoms in chronological order. In other words, it starts with the earliest signs and moves through to the most recent signs.

Algae growth

green algae low ph in aquarium

Green algae growth is the first sign that your aquarium’s pH level is low. Algae thrive in alkaline water. The increased amount of available nutrients and light encourages algae to grow. Algae is usually green, but it can also be brown or dark in color. Green algae are the most common type of aquarium algae.

If the algae start to take over your aquarium, your fish will be more vulnerable to getting sick. Furthermore, algae can cloud the water, making it harder to see your fish swim around. Algae is unsightly and lowers the water’s aesthetic value – which is why most aquarium owners want to keep algae under control.

Sludge and dirt build up on the bottom of the fish tank

Sludge appears when fish eat fish food, and it sinks to the bottom. The fish food breaks down in an oxygen-poor environment and turns into what looks like gray mud or sludge. This sludge prevents water flow, clogs up your filter, and releases a foul odor into your tank. It can also cause stress for your fish.

If you don’t clean it up, the sludge will only get worse. However, if you clean it up too much, you can remove too much of your tank’s beneficial bacteria – which means your tank’s water quality will suffer.

Symptoms of bad pH in aquariums

Low pHHigh pH
Pale or dark fishTiny white spots on fish (Ich)
Green algaeFish scratching against rocks
Stressed out fishGreen algae and slime
Fish going to surface to breatheSluggish behavior in fish
Strange movementFast and uncontrolled movement
Staying at bottom of aquariumFish going to surface to breathe

Effects of Low pH on Fish

Stressed fish

Fish are stressed when their tank water has low pH levels. This stress is similar to being in an environment that’s too hot or too cold. It decreases the fish’s ability to swim, eat, and reproduce. If low lighting or overcrowding stresses out your fish, you will start to notice their behavior change. For example, they will act sluggish and begin sleeping more often.

Fish laying on the bottom

If your fish is at the bottom of the tank or close to the surface, it’s a sign that they have trouble breathing. Low pH levels make it more difficult for fish to absorb oxygen through their gills.

When they are struggling to breathe, they will naturally gravitate towards the surface or bottom of the tank. It is a behavior that they naturally exhibit in water, where there isn’t a lot of oxygen.

Pale-colored fish

When alkalinity levels drop, your fish’s metabolic rate slows down, and their body goes into a state of autophagy (self-eating). It means the animal’s cells start breaking down. The metabolic rate slows down, and its color becomes paler.

If you notice this change in your fish, don’t be alarmed – it’s just their body trying to adapt to the changes in its environment. As alkalinity levels improve, the fish will go back to normal.

Related article: pH too high? Read our guide on how to lower pH using baking soda

Symptoms of Low pH in Fish Tank

What Causes Low pH in Aquarium?

If any of these elements are missing from your aquarium water, they will cause your fish tank’s pH level to be too low.

 1.) Lack of Calcium and Alkalinity

These two elements together are necessary for the formation of buffers and a healthy pH level. When a fish tank lacks calcium, its pH will drop. The alkalinity level also affects the pH of your aquarium water. When it’s too high, your fish’s swimming ability will suffer – which can lead to stress and eventually death.

2.) The filter isn’t cleaning the water properly

For a tank’s water quality to stay good over time – its filter needs to be in good condition. Without a working filter, your tank will have a buildup of waste on the bottom of its tank and algae on its glass walls. In addition, you need to clean the filters regularly – which means you’ll need a powerful filter that can handle a lot of waste.

Related read: How to change your filter without losing important bacteria

3.) Not enough plants

Plants help stabilize the pH of your fish tank because they remove toxins from the water. However, you can’t just place any plant in an aquarium and expect its leaves to absorb all the harmful substances in the water. Various plants eat fish waste, but most aquarium owners don’t get this information from the local fish store. It would help if you, therefore, did extensive research before you buy any plant.

4.) Too many fish

The more fish you have in a tank, the more food you will need to feed those fish daily, which means you will add more waste to your tank. Fish waste is usually a source of ammonia and nitrites. If your fish tank becomes too crowded, its pH level will drop. In addition, the waste could be so overwhelming that it causes a fish to start dying.

Related: How many fish can you put in a 3 gallon tank?

How Do You Test Your Fish Tank’s pH Level?

The easiest way to test your aquarium’s pH is to use an inexpensive electronic testing kit. Find one that’s relatively easy to use and gives you results quickly. Some kits require you to use tiny drops of liquid, while others require you to dip the tester into your tank’s water.
Before you begin testing, make sure your aquarium is at least half-filled with water. Also, make sure it has an air stone running in it. Running water encourages the fish to stay near the surface – making it easier to breathe if they are stressed out.

How Do You Adjust Your Fish Tank’s Water Chemistry?

You have to adjust the pH level of your fish tank if the symptoms are too severe. However, keep in mind that some aquarium fish are more sensitive than others. So, if you lower the pH level too far, some fish will become very ill and eventually die. Typically speaking, most fish can handle a drop in pH by no more than 1-2 points – but this varies depending on the fish itself. For example, cichlids do better when their water has a higher concentration of carbon dioxide.

How Do You Monitor your Fish Tank’s pH Level?

The best way to monitor the pH level of your aquarium is to test it regularly. This way, you can keep tabs on your fish tank’s water – and keep any issues from getting out of hand. Also, keeping track of the level over time will help you figure out what triggers a rapid change in your fish’s environment so that you can avoid those situations altogether.

If the pH level drops too low, you’ll notice your fish starting to become sick and even die. If this happens, check your fish’s temperature and if that is stable, then go ahead and treat the tank with a complete set of salts. If its temperature rises, then you do not need to treat your fish tank.

If the pH level gets too high, you’ll notice your fish’s behavior is different. For example, if they stop swimming and hiding and start floating around and looking for food – this usually indicates the fish is stressed out and needs treatment. The good thing about this is that once you adjust your tank’s water chemistry to a setting that’s safe for them, the fish will remember what they are used to and will continue to thrive.


Jason Matthews

My name is Jason Matthews, and welcome to my website. When other kids were bragging about how their dog could sit and roll over, I was bragging about my latest Betta Fish and the cool sea castle I just added to his aquarium. 

Jason aquariume

🐠 Get Your Free Ebook: A Beginner's Guide to Aquariums - Sign Up Now!

Learn everything you need to know to keep fish, including setting up your tank, choosing the right fish, and maintaining water quality. As a subscriber, you'll also stay up-to-date on the latest aquarium products, special offers, and discounts.

Your privacy is important to us. You can trust that we will never share or sell your information, and we'll only send you relevant content.

Leave a Comment