Is pH important in a planted aquarium ?

Above: Neocaridina shrimp (Bloody Mary) in a 2 hr tank. They reproduce wildly in planted tanks, even though the daily pH fluctuations in such tanks can easily be more than 1 point.

What is pH, really?

pH is a measure of how acidic or alkaline the water is. The best way to think of pH is as the ratio of positive electrical charges from ionized hydrogen cations (H+) to the negative electrical charges from hydroxyl anions (OH-). When there is exactly the same amount of (H+) and (OH-), all the positive charges are balanced by all the negative charges. A pH reading of 7 is neutral, while numbers below 7 are acidic and numbers above 7 are alkaline.

pH swings in nature

Fluctuations in pH caused by changes in carbon dioxide (CO2) concentrations are common in many natural bodies of water. CO2 builds up overnight in natural bodies of water due to organic decomposition. This causes the pH of the river/lake to drop. As soon as sunlight hits, plants begin to remove CO2 from the system as part of photosynthesis. This causes the pH in the water to rise;

pH swings of up to 1 full point are therefore common in many natural lakes/rivers; this usually occurs quite rapidly between dawn and noon, as vegetation rapidly removes carbon dioxide from the water when the sun is up.

Neither fish nor plants living in these natural waters are affected by these changes. (So much for the myth that pH stability is all that matters in a tank).

Taken from Allen HL. 1972 Phytoplankton photosynthesis, micronutrient interactions in a soft water Vermont lake

There are certain species of fish that come from more isolated waters that do not experience as much fluctuation in water parameters. However, it is still a good idea to check the species you are keeping, especially if they are rare or wild-caught.

However, most of the animals commonly kept in the aquarium trade are completely unaffected by small fluctuations in water pH. Planted aquariums are now commonplace, with thousands of examples around the world. In CO2-injected planted tanks, most use a timer to inject CO2 only when the tank lights are on; since CO2 is only used by plants when there is light. In these tanks, the CO2 would cause a pH drop of 1 to 1.5 points. Yet we see thriving fish and shrimp in these tanks.

Will a pH of less than 6 kill the bacteria responsible for tank cycling? 

The short answer is no. Many natural rivers have pH in the 4 to 5 range and they are teeming with life, both fish as well as bacteria. For folks that want a more technical explanation of how bacteria can adapt to low pH conditions, this scientific journal is a good read.

KH is what really matters

pH is closely related to carbonate hardness (KH), which measures the amount of carbonate (CO3) and bicarbonate (HCO3) ions in water. Most waters contain carbonates - and the greater the concentration of carbonate ions, the higher the pH and the more resistant the pH is to factors that would lower it. For example, if Tank A is located in a region where the tap water has a KH of 0.5 degrees, while Tank B has tap water with a KH of 7 degrees, if a small amount of acid were added to both tanks, the pH of Tank A would drop much faster than that of Tank B.

We can also change the carbonate hardness (KH) level in the tank, for example by adding limestone, which increases the carbonates in the water. Increasing/decreasing KH will always result in an increase/decrease in pH. However, the reverse is not true - we can change the pH level in a planted tank without significantly changing the KH level. The prime example of this is CO2 injection, which acidifies the water and lowers the pH, but does not change the carbonate hardness (KH) of the water.

KH fluctuations affect livestock because changes in carbonate salt concentrations affect osmotic functions in livestock. This flux will also show up as changes in pH. Thus, pH changes due to KH changes will affect livestock, but pH changes without KH changes do not have the same impact.

To this end, KH stability is much more important in aquariums than pH stability. The stability of the latter (pH) is only important as an indication of the stability of the former (KH).

So why all the emphasis on pH?

It is more an effect of historical precedence than anything else; pH is easy to test and understand, while testing for KH requires titration. With the improved understanding of today's science, we should be shifting our emphasis to paying more attention to KH rather than pH because that is what ultimately affects the animal/plant.

So, one can ignore pH totally?

This is not to say that the importance of pH is completely negated, especially for certain sensitive animals. However, in almost all such cases, animals that specifically require a certain pH range will also require a correspondingly specific KH range, with the latter taking precedence. For example, you can keep African Cichlids in a planted tank with CO2 injection (which lowers the pH close to 7) as long as you maintain high KH levels in the water.

Fish from acidic peat swamps will do well in a tank where the pH drops from 7 to 5.8 during CO2 injection because the low point of the cycle pH (5.8) is within the range of their natural living conditions. However, alkaline water fish may not tolerate the dip into the pH 5.8 range well. It is not the fluctuation, but the pH itself that is outside the natural range of the fish. The same alkaline water fish can survive a comparative pH swing from 8.5 to 7. It is important to distinguish the effects of a pH change from the effects of an extreme pH in and of itself.

pH swings due to CO2 changes are not impactful. However, extreme values of pH, high or low, whether caused by CO2 or other acids can affect livestock.

Sundadanio axelrodi have a reputation for being very sensitive fish - this batch has thrived in a 2hr CO2 injected tank (with daily pH swings of more than 1 degree). Their deep blue/red coloration compared to what you usually see in pet stores is a testament to favorable tank conditions.

What pH works for fish?

A normal range in which most commercially available animals and plants can survive would be between pH 6.0 and 8.0. For most tanks, pH levels will not fluctuate to the point of being harmful as long as the KH is kept stable. Fish from regions with higher or lower pH levels may be more suited to the outliers in this range, e.g. fish from acidic peat bogs may do well at pH 4.0, while many cichlids from alkaline water lakes are comfortable in higher pH ranges (8-9+) but do not prefer acidic pH ranges.

If you are keeping fish that are less tolerant of low pH, buffering the water to higher KH levels will prevent the pH from dropping too low due to CO2 injection. For most general community tanks, 0.5 - 2dKH of alkalinity is more than sufficient to prevent the water from becoming too acidic to affect the animals.

Why is pH so often quoted as such an important criteria if it is not really so?

This is a holdover from old aquarium science when pH test kits were the only commonly available form of testing. It was useful as a proxy for KH because KH fluctuations affect pH readings - generally speaking, a stable pH means that KH is stable (which is important). A KH fluctuation of more than 3 degrees in a short period of time can easily harm sensitive animals like shrimp. And keeping KH stable is what is important. Today, we can measure KH, GH, TDS, and pH separately, and our understanding of which variables affect livestock has improved. 

So what pH is ideal for a planted tank?

A pH of 6 to 7 - slightly acidic, is what most planted tanks function well optimally at, and this is also why all aquasoils contain peat which slightly acidifies the tank and lowers KH levels.

Common plant species that are not picky can be grown in much more alkaline water pH8+. To narrow it down and be more specific where picky species are concerned, read the article on KH (carbonate hardness) as that takes precedence over pH.

As Singapore has very soft tap water, most non-limestone tanks have a KH of less than 1; and the pH swings more than 1 degree in these tanks when CO2 is on/off.