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 several reasons why pH matters. First of all, many aquarium fish have spent their entire lives in water with a particular pH. This is especially true of many wild-caught freshwater fish and those bred overseas. When these fish are moved from the aquarium store to your aquarium, the sudden change in pH often results in losses due to shocked fish.

A pH shifting up or down can be a sign of underlying problems. Most organic matter tends to be acidic, so dirty, poorly maintained aquariums often 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 kill aquarium inhabitants.

In Reef Aquariums, depressed pH inhibits calcification and can significantly reduce 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 several ways. Colorimetric tests are the most common way. With these tests, you add a colored reagent to a vial of aquarium water, then compare the color to a chart to determine the pH. The MarineAndReef.com pH Tester is the perfect all-in-one multi-range tester. The API pH Test Kit and API High-Range pH Test Kit are good basic tests. Make sure to purchase the correct range for your aquarium type (ideal pH ranges specified later).

A more advanced option is to use a pH probe. 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.

-Check out our MarineAndReef.com pH Tester Tutorial YouTube Video to see this visually.

What is the proper pH?

There is considerable debate about the ideal pH range. The ideal and acceptable ranges for different aquarium types are listed 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.

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, you can switch to a different source or add an acid to lower the pH of the source water. We recommend Seachem Acid Buffer. Every time you add water, mix the acid buffer into the new water to adjust the pH to your desired value, so the water going into the tank has the pH you want.

If your tank pH is much higher than your source water, it's often caused by additives or decorations that raise pH. Read the labels of all the additives you are using to make sure they do not raise pH, and check for any calcium-based decorations in the aquarium. This includes seashells 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 low pH, and how can it be fixed?

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 still does not reach your desired level, it is likely due to elevated carbon dioxide in the room where you are mixing the saltwater. Increasing ventilation in the room where you are mixing seawater 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 aquarium’s sand/gravel will produce sulfuric acid. Do water changes with properly adjusted pH source water while vacuuming out as much 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 lot of fish, it is likely the CO2 is coming from the aquarium. Ventilating the aquarium will help equalize the CO2 in the aquarium with that in 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 room's air has really high CO2 levels.