Being the proud owner of a well balanced aquarium often comes with a lot of learning about water chemistry and test water results. One of the most important values you need to keep track on any aquarium is the pH. Whether your aquarium is a freshwater tank or a marine coral reef, your fish will be happier when you keep a stable pH value. Sometimes this means that you will need to raise or lower the pH in your aquarium artificially. It is extremely important that this process is done slowly, as brusque changes in pH can kill your fish, or at the very least make them severely unhappy and more likely to fall prey of sickness and parasites such as ich. Besides pH, another measurement to keep in mind is the KH or carbonate hardness which acts as a buffer to prevent brusque changes on pH.
What is pH?
pH is a measurement that indicates how acidic or alkaline a solution is, using a logarithmic scale where a 7 is the value for neutral (neither acid, nor basic). Being logarithmic means that is much easier to change the pH from 1 to 2 than it is to change it from 6 to 7, for example. Water with a pH of 5 is 10 times more acidic than a pH of 6, which is why small changes of pH at higher ranges can be much more noticeable.
The pH measures the amount of free hydrogen ions available in the solution, and as such is affected by other water parameters such as the carbonate hardness (KH) and the general hardness of the water (GH). Pure water is neutral, but water on an aquarium will have chemicals dissolved on it that will change its pH to either acidic or basic (alkaline). A saltwater aquarium will be alkaline, whereas a freshwater aquarium with very soft water (low GH and low KH) will be pretty acidic, and suit totally different kinds of fish.
Since fish are fully immersed in water, changes in pH or incorrect pH values on their environment are often a deadly matter. Fish are, however, very adaptable creatures that will be able to survive high variations in pH (6.5 to 8.2) but by regulating the amount of minerals they take from the environment however a sudden change in pH can have disastrous effects. If you are raising or lowering your aquarium pH it is important to do it slowly so fish can acclimatise.
Freshwater Fish Recommended Aquarium pH Ranges
Most tropical fish will survive pH ranges between 6.5 and 8.5, however the closer you are to their natural environment the happier your fish will be. The exception are fish that require soft water (acidic water) and fish that require hard water (alkaline water). This is because water hardness is directly related to the pH of the aquarium, and the other way around. Since lowering the pH of hard or basic water is very difficult due to the high KH values offering a lot of buffering space, it is better to avoid soft water loving fish on hard water zones if you don’t have access to RO water and plan on using tap water for your aquarium.
If you have very soft (acidic) water, your best options are Pikeheads, Ram Cichlids, and Chocolate Gouramis, which love pHs as low as 5.5. However you will find that your biological filter won’t work as well, as the bacteria require a pH between 7 and 8 to thrive. Tetras, Raspboras and Discus also thrive in low pH aquariums, along with West African and South American cichlids. Be careful when choosing your cichlid species, as Malawi cichlids only survive in alkaline environments.
Most tetras, silver dollars, loaches and most algae eaters will be fine on pHs around 7.0 to 7.5, provided the water is soft (low GH measurement). On the other hand, aquariums with hard water will be perfect for danios, gouramis, catfish, rainbow and redtail sharks, guppies, platies and swordtails. Many cichlids (non West African ones though) can also thrive in this conditions.
Mollies, Malawi Cichlids and Tangynika lake Cichlids will need hard, alkaline water, with a pH of 7.6 to 8.6. This kind of fish are better kept on a species tank instead of a community tank.
Recommended pH on Marine Aquariums
A fish only marine tank can run on pH levels between 7.6 and 8.4. If you choose to introduce corals and what is known as a reef aquarium pH and water chemistry become much more important. You will need to keep the pH between 8.0 and 8.4 for corals to be able to use calcium to grow and for fish to be healthy.
Due to marine aquariums being at the higher end of the pH spectre the changes on pH are usually less sudden, and regular water changes will help the aquarium buffers to work at full efficiency. However, since a change in the pH conditions can be deadly very quickly it is important to be prepared to raise or lower the pH on your marine aquarium when it goes out of the required parameters. In an emergency you can use sodium bicarbonate at a rate of one teaspoon per 20 gallons to increase the pH, or vinegar or CO2 injection to lower it. However, do it slowly or the cure could prove worse than the original problem.
How To Raise Aquarium pH Safely
If your aquarium pH is too low, adding decorations or substrates that leach minerals into the water can safely raise the aquarium’s pH over a period of time. For example:
- crushed corals, which are available in most aquarium supplies stores
- add dolomite to your filter or substrate
- choose limestone (calcareous rocks) to decorate your aquarium
All the above materials have in common releasing calcium on the water, which will raise the water hardness and the pH. They are commonly used on saltwater aquariums where a high pH is needed, and will also increase the buffering ability of the water.
Another way to raise aquarium pH is by growing plants, which will dispose of the excess CO2 present in the water and help lower the pH. However, most planted aquariums will need CO2 injection to achieve optimum growth and prevent the formation of algae colonies, so this method is mostly useful for marine aquariums.
There are chemical buffers that can be used to raise the aquarium pH levels and keep it stable at a particular number (hence the name buffers) however you may find that adapting the decor and inhabitants of your aquarium to encourage higher pH levels is more cost effective.
How To Lower Aquarium pH Naturally
In order to lower fish tank pH levels you will need to soften the water and acidify it. Lowering aquarium’s pH permanently is more difficult than increasing it, so before you choose to acquire fish species that require acidic and very soft water it is a good idea to test the natural pH and hardness of the water in your area. If you have access to RO water, using it for water changes will reduce the overall pH of the water.
Other methods you can try are:
- Filtering through peat moss, using it on your substrate or even letting your water stand on a tank with peat moss for a couple of weeks will make the water more acidic. However, the moment you stop using it the water pH will raise again if your tap water is hard and you don’t use RO water.
- A CO2 injector will lower the pH on the aquarium as some of the CO2 becomes carbonic acid. This can be desirable on planted aquariums as it also helps algae control.
- Driftwood will lower the pH on an aquarium slowly over a period of time, but it can make the water take a yellowish tinge due to the tanins it contains.
- pH buffers can be used as additives to lower the pH on an emergency, but they won’t keep the pH down. When you stop using them, you risk the pH bouncing right back up in a deadly pH surge.
If you live in a hard water zone, keeping your aquarium water pH low enough to host soft water fish will be an uphill battle, which will most likely require using RO water or distilled water on every water change. Left to its own devices, water will go back to whatever pH the water you use for water changes has, which means adding additives, CO2 or peat moss on every change. The effects of driftwood are not very noticeable, but if your water is not very alkaline it will allow you to keep acidic water loving fish such as Discus.
Generally speaking, raising or lowering your aquarium pH should be done only on exceptional circumstances, and over a long period of time so the fish can get used to it. Since keeping pH levels significantly higher or lower than your tap water is pretty time consuming and can have terrible consequences, it is always better to try to only purchase fish that are actually happy on the pH range your water naturally has. Otherwise pH swings are a risk that needs to be carefully monitored.