Why is My Aquarium pH so High/Low?
How To Achieve the Proper Aquarium pH

PH is one of the most often tested aquarium water parameters. It is also a common cause of confusion and headaches. Here is what you need to know about pH and pH testing for your aquarium.

What is pH?

Simply put, pH is the ratio of acids to bases in your aquarium. Adding acids or removing bases lowers pH, and adding bases or removing acids raises pH. The lower the pH the more acid in the water, and the higher the pH the more bases in the water. Common acids in aquariums include carbon dioxide, humic acid, citric acid, Vinegar, and sulfuric acid. Common bases include sodium hydroxide, calcium hydroxide, calcium carbonate, and magnesium hydroxide.

Why does pH matter?

There are a variety of reasons why pH matters. First of all, many fish in aquariums have spent their entire life living in water with a particular pH. This is especially the case with many wild-caught freshwater fish and freshwater fish bred overseas. When these fish are moved to the aquarium store and then to your aquarium the sudden change in pH will often result in losses from shocked fish.

A pH shifting up or down can be a sign of underlying problems. Most organic matter tends to be acidic so often dirty and poorly maintained aquariums tend to have a depressed pH. An elevated pH can be a sign that some additive was overdosed. Too much of a pH swing in either direction can lead to the death of aquarium inhabitants.

For Reef Aquariums, depressed pH inhibits calcification and can greatly decrease coral growth. The growth difference can be substantial. Sometimes depressed pH can cut a coral’s growth rate in half.

How to measure pH?

PH can be measured in a variety of ways. Colorimetric tests are the most common way. With these tests, you add a colored reagent to a vial of aquarium water and then compare the color to a chart to find the pH value. The API pH Test Kit and API High-Range pH Test Kit are good basic tests. Make sure to purchase the correct range based on what kind of aquarium you have (ideal pH ranges specified later).

A more advanced option is to use a pH probe. A pH probe connected to a pH monitor such as the American Marine Pinpoint pH Monitor will give you a constant digital display of the aquarium’s pH. An aquarium controller such as a Neptune Apex or
CoralVue Hydros can monitor a pH probe and provide a constant pH feed to your smartphone. It can graph the pH over time, and alert you if the pH goes out of range.

What is the proper pH?

There is a large amount of debate about the ideal pH range. The ideal and acceptable range for different aquarium types is below. With the ideal pH, your corals and fish will get better coloration and growth, and they will have a higher chance of breeding and reproduction. With the acceptable range, your animals are not at risk of dying, but you will not have the healthiest aquarium.
  • South American Fish and Discus
    • Ideal 5.5-6.5
    • Acceptable 5.0-7.5
  • African Fish
    • Ideal 7.4-8.0
    • Acceptable 7.0-8.5
  • Freshwater Community Tanks
    • Ideal 6.5-7.5
    • Acceptable 6.0-8.5
  • Reef Aquariums
    • Ideal 8.3-8.5
    • Acceptable 7.8-8.5
  • Saltwater Fish Aquariums
    • Ideal 8.0-8.5
    • Acceptable 7.7-8.5

What causes a high pH and how to fix it?

The first thing you should do when addressing any pH problem is test your source water. This would be your tap water, or RO water for freshwater tanks, and your mixed saltwater for saltwater tanks. In most cases, your tank pH should be similar to that of your source water. If the pH of your tank matches your source water but is still higher than you would prefer, then you can switch your source water or add an acid to lower the pH of the source water. We recommend Seachem Acid Buffer. Every time you add water you should first mix in the acid buffer into the new water to drop the pH to your desired value so that the water going into the tank has the pH you want.

If your tank pH is much higher than your source water then this is often caused by additives that raise pH or decorations that are raising pH. Read the labels of all the additives you are using to make sure they do not increase pH and make sure that there are no calcium-based decorations in the aquarium. This includes sea shells and aragonite gravel. If you find anything that could be raising the pH, stop using it and the pH will naturally drop over time.

What causes a low pH and how to fix it?

Just as with a pH that is too high, start by testing your source water. If your aquarium pH matches your source water but the pH is still lower than you like, consider adding a buffer to the water. For freshwater tanks, we recommend Seachem Alkaline Buffer. If your premixed saltwater has a pH that is lower than you would like, it is most likely not aerated well enough. Use a power head to stir and aerate the seawater to encourage the pH to go up. If the pH will still not go as high as you like then this is likely due to elevated carbon dioxide in the room you are mixing the saltwater in. Increasing ventilation in the room you are mixing seawater in will help to raise the pH. Once you have source water with a proper pH, start using it and this may solve your pH problem.

If your source water has the pH you want and your tank pH is still low start by cleaning the tank. Detritus and organic waste will release acids as it decomposes and anaerobic bacteria that grow in many stagnate neglected areas of the aquariums’ sand/gravel will produce sulfuric acid. Do water changes with proper pH source water while vacuuming out as much of the detritus as you can. This can make a bigger effect on the aquarium's pH than most people think.

Once you are confident that the aquarium is clean and free of detritus, if your pH is still low then this is most likely due to excess carbon dioxide (CO2). CO2 comes when animals respire. This includes your fish as well as any people or animals living in your home. If you have a whole lot of fish, then it is likely that the CO2 is coming from the fish in the aquarium. Ventilating the aquarium will help to equalize the CO2 in the aquarium to that of your room. Removing glass lids, adding air pumps, pointing powerheads at the water surface, and using cooling fans to increase evaporation will all help to ventilate the aquarium and equalize the CO2 levels between the aquarium and your home’s air. However, this won’t do much good if the air in the room has really high CO2.

If your aquarium is in a room with high levels of CO2, it can lead to chronic low pH. There are a few things that can help with this:
  • Opening windows when possible, using an air exchanger, and making sure you use the vent hood when cooking can help to reduce CO2 in your house.
  • Refugiums and algae turf scrubbers will help with CO2 as algae remove CO2 as part of photosynthesis.
  • Switching your reef aquarium supplements to products that boost pH such as Kalkwasser and Brightwell Aquatics Boost pH+.
  • Put a CO2 scrubber on the air inlet of your protein skimmer. We offer an IceCap CO2 Scrubber and Two Little Fishies CO2 Scrubber Bundle.
In most cases addressing low pH from high household CO2 levels will require a multipronged approach where a combination of mitigation techniques will be needed.