Symptoms Of Low pH In A Fish Tank


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Algae growth, sludge/dirt at the bottom of the fish tank, sudden changes in your fish’s appearance and behavior, are all signs that your aquarium’s pH (alkalinity) has dropped too low.  

Potential hydrogen (pH) measures how acidic or alkaline/basic a solution is, with a numeric scale ranging from 0-14 with 7.0 being neutral, <7.0 acidic, and >7.0 being alkaline or indicating a base. 

It is important to measure pH in a fish tank to understand how acidic or alkaline the aquarium water is. Knowing the pH of tank water is vital for fish growth, and their overall health. 

While every aquatic species has a specific pH tolerance level, large fluctuations can be dangerous for fish and other life inside your fish tank, therefore, it is essential to know the symptoms of low pH shock and how to identify them, as low pH is linked to poor breathing and agitated gills in fish.

Symptoms Of Low pH In A Fish Tank

If your fish tank is suffering from a low pH level, you can expect to see the following symptoms. 

1. Algae Growth 

As algae thrive in slightly acidic water, algae growth is often the first sign your fish tank has a low pH level. With more nutrients available and accessible light, algae can spread like wildfire when pH levels drop. 

Even though green algae is the most common type of aquarium algae, algae growth can also be brown or even dark in color. 

If you notice algae spiraling out of control, your fish are more likely to become sick. Excessive algae also makes it difficult for your fish to swim around the tank. 

2. Dirt Build-up At The Bottom Of The Tank

Dirt, often referred to as “sludge” in aquariums, appears when uneaten food sinks to the bottom of the fish tank. As the food decomposes, it turns into a gray sludge because the bottom of the fish tank is an oxygen-poor area. 

While the sludge may not look like it is an issue, it can prevent water flow, block the tank filter, and release a bad smell into the tank, all of which will stress your fish. 

If you leave the sludge inside your fish tank, it will quickly build up and the problems will get worse. However, when it comes to removing it, be careful not to remove too much as the sludge contains the fish tank’s beneficial bacteria which actually helps the overall water quality. 

3. Stressed Fish

When the pH inside your fish tank is too low, your fish will easily become stressed. Imagine being in a room that is far too hot, or far too cold, I am sure you would soon start to feel very uncomfortable! Well, this is how your fish feel when the pH is too low inside their home. 

Low pH reduces your fish’s ability to swim, eat, reproduce, and grow – everything it must do to survive. If your fish become stressed, they will seem sluggish, often sleeping more than usual. 

4. Fish Laying On The Bottom Of The Fish Tank, Or Hanging-out At The Surface

When fish spend most of their time at the bottom of the fish tank or close to the surface, this is a sign they are struggling to breathe, because of acidic conditions. 

Acidic conditions make it very difficult for your fish to absorb oxygen via their gills. The struggle to breathe from lack of oxygen causes them to naturally gravitate either towards the top or the bottom of the fish tank. 

5. Pale-looking Fish

When fish become paler than normal, this is a prime example of an alkalinity drop. Immediately when this happens, your fish’s metabolism slows down and their body suffers from autophagy; an adaptation to prevent nutritional starvation (self-eating). As this happens, your fish’s cells start to break down and they appear more pale than normal. 

If this happens to your fish, do not worry too much – this is a normal process as they try to adapt to the environmental changes. As the alkalinity level improves, your fish will soon recover. 

Generally, alkalinity in fish tanks should be 5.5-7.5 for freshwater fish tanks, and 8.0-8.1 for saltwater fish. 

Symptoms Of High pH In A Fish Tank

You should also be aware of symptoms associated with a high pH in your fish tank. 

  • White spots on your fish, known as Ich.
  • Fish rubbing against rocks/substrate. 
  • Green algae and slime inside the fish tank.
  • Sluggish behavior from your fish. 
  • Fish moving uncontrolled and swimming erratically. 
  • Fish gasping for air at the surface. 

Causes Of Low pH In A Fish Tank

If you are already an experienced aquarium hobbyist, you will know how difficult it can be to replicate your fish’s water requirements. Many elements that affect the pH in the wild can also affect the pH inside a fish tank. 

The issues associated to low pH in fish tanks are stated below:

  • Too many fish
  • Overfeeding
  • Tank under-cycling
  • A dirty tank
  • Water source when performing water changes
  • Decaying fish
  • Contaminated or sick fish
  • Aquarium gravel/substrate hiding waste
  • Not enough plants

How To Test & Monitor pH In A Fish Tank?

Maintaining water quality is key to keeping fish alive in a fish tank. The pH from your tap water may differ from your neighbors, so testing water and knowing how to alter it, is super important to reduce stress inside the aquarium. 

Aquarium experts suggest removing some aquarium water into a separate container to aerate with an air stone for a few days before testing. This is because “true” pH can be inhibited by CO2 (carbon dioxide) levels inside your fish tank, causing the pH levels to read lower than they actually are. You wouldn’t want to try and adjust the pH levels prematurely, as this could further complicate your low pH issue. 

The most accurate way to measure pH in aquariums is with a pH probe/sensor attached to a pH meter. There are many different types of pH sensors, however, a pH sensor for everyday use like the Atlas Consumer-grade pH Sensor (or the Atlas Lab-grade pH Sensor if you are looking for a high-accuracy monitoring kit) is recommended. 

For high-accuracy measurements, it is recommended to calibrate your pH probe before each use. 

Alternatively, you can use pH paper test strips, also known as Litmus paper. These are very cheap and an easy way to test the pH inside your fish tank, however, they are not as accurate as pH meters.

As pH can vary during the day, you should always test the pH levels in the morning or evening. 

Even though dissolved oxygen (DO) does not directly affect pH levels because there is no physical-chemical connection between the two, in some cases there are indirect relationships from external factors, such as additional nutrients increasing algal growth, therefore it is also recommended you measure DO levels inside your fish tank with a DO probe and sensor.

How To Adjust pH In A Fish Tank?

It is always recommended to try and acclimate your fish to the pH level that is already inside your fish tank before you start “fiddling” with pH levels. However, some people prefer to increase or decrease the pH levels to match exactly what is recommended for that fish species. 

Ways To Safely Raise The pH Level In A Fish Tank

When the pH drops inside your fish tank, I’m pretty sure you would react like any other fish enthusiast and panic. It is important to wait and think about your approach, as irrational decisions often end up as a disaster! 

Safely raising the pH levels in your fish tank will prevent unwanted stress for both you and your fish.

Water Changes

Water changes are something every aquarium hobbyist should already be very familiar with. The most effective method to increase the pH level in a fish tank is to perform regular water changes and top up with tap water and a water conditioner. 

If you do not perform regular water changes, it is recommended you only perform small water changes rather than one large one, as this reduces the chances of your fish suffering from shock. 

Adding Rocks & More Substrate

Everyone loves a bit of aquascaping! An aesthetically-pleasing way to increase the pH in a fish tank is to add more rocks or substrate to your aquarium. 

Crushed coral is a great choice! But, if you cannot find any crushed coral, limestone also works the same, as coral is made up of calcium carbonate, similar to limestone. Remember to always add limestone or crushed coral gradually, as this method can quickly increase the pH level in water if you overdo it. 

Adding shells is another easy way to increase the pH of water in fish tanks and gives that “mini-ocean-look”. 

Increasing Oxygen Levels

By increasing the oxygen (O2) concentration inside your fish tank, the more aerated the water will become, and therefore, the carbon dioxide (CO2) concentration is driven down. 

We recommend checking your aeration device, as very large tanks require a more powerful device than basic ones on the market. For best results, check your DO and read about how DO influences water quality.  


A common method is to use baking soda. Although you shouldn’t be tempted to use over-the-counter remedies to buffer your fish tank, some hobbyists have been successful with this method. 

Adding Chemicals

As a last resort, you can try adding chemicals like commercial buffers. 

This is not recommended unless you are really struggling, as adding chemicals can lead to large spikes in pH, and generally, chemical treatment is a temporary fix to the problem. 

However, if you do go for this option, always use safe buffering agents and a product you know won’t magnify your water chemistry issues.

Ways To Safely Lower pH Levels In A Fish Tank

Lowering the pH inside a fish tank is often more difficult than trying to raise it, however, do not worry, we still have a few tips to lower pH levels inside your aquarium.

Filtering Peat Moss 

Firstly, we recommend getting your hands on some peat moss. Filtering peat moss is the most successful way to lower your fish tank’s pH levels. 

Some aquarists also use peat moss in their substrate or add it into a mesh bag. However, do note that your water may temporarily discolor, but do not worry, this will soon clear up. 

Adding Wood

Adding wood like driftwood will also lower the water’s pH. Wood not only helps lower pH levels in your fish tank, but it also looks great, and your fish will also appreciate the extra hiding spaces and areas to explore! 

Increasing CO2 levels & Adding RO Water

Pumping or increasing carbon dioxide levels (via plants) or adding RO (reverse osmosis) water are also easy ways to reduce pH levels.  


Generally, every fish tank should have a neutral pH (7.0). If this drops too low or rises significantly, it can be dangerous for the fish’s health and compromise the water quality inside the aquarium.

However, always remember that stability is the key factor here. Fish can adapt to a pH that is slightly off the range. It is large swings in pH levels that cause stress and death in fish. 

If you have any questions regarding pH or the pH probes we have to offer, please do not hesitate to contact the world-class team at Atlas Scientific. 

pH Probes & Sensors

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