Above: Neocaridina shrimp (Bloody mary) in a 2Hr Tank. They breed wildly in planted setups, even though the daily pH swings in such tanks is easily 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 about pH is as a ratio of positive electrical charges due to ionised Hydrogen cation (H+) versus the negative electrical of Hydroxyl (OH-) anions. If there are exactly the same amount of (H+) and (OH-) then all the positive charges are balanced by all the negative charges. A pH reading of 7 is neutral, while numbers lower than 7 are acidic, and number higher than 7 are alkaline.
pH swings in nature
pH fluctuations that stem from from changes in Carbon dioxide (CO2) concentrations are common in many natural bodies of water. CO2 builds up in natural bodies of water overnight due to organic decomposition. This causes the pH in the river/lake to fall. Once sunlight hits, plants begin draining the system of CO2 as a part of photosynthesis. This corresponding causes the pH in the water to rise.
pH swings of up to 1 full point is thus common in many natural lakes/rivers; this usually takes place quite quickly between dawn and noon, as vegetation quickly strip the water of carbon dioxide when the sun is up.
Neither fish nor plants that live in these natural waters are affected by these changes. (So much for the myth that pH stability is everything in a tank)
Taken from Allen HL. 1972 Phytoplankton photosynthesis, micronutrient interactions in a soft water Vermont lake
There are specific fish species that come from more isolated water bodies which do not face as much fluctuations in terms of water parameters. So it still make sense to check up on the particular livestock one is keeping; this especially so for rare or wild caught species.
However, most commonly bred livestock in the aquarium trade are totally unaffected by small swings in water's pH. By now planted aquariums are common place with thousands of examples across the globe. In CO2 injected plant tanks most use a timer controller to inject CO2 only when the tank's lights are turned on; as CO2 is only utilized 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 such 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 tied closely to carbonate hardness (KH) which measures the amount of Carbonate (CO3) and bicarbonate (HCO3) ions in the water. Most water contain carbonates - and the greater the concentration of carbonate ions, the higher the pH is and the more resilient the pH is to factors that would lower it. For example, Tank A is in a regions where tap water has a KH of 0.5 degrees whereas Tank B has tap water of KH 7 degrees; if both tanks had a small amount of acid added, Tank A's pH would fall a lot faster than Tank B.
We can also change the carbonate hardness (KH) level in the tank, for example by adding limestone, which increases carbonates in the water. Increasing/decreasing KH will always result in an increase/decrease in pH. However, the converse 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 drops the pH but does not change the carbonate hardness (KH) level of the water.
KH fluctuations affect livestock as changes in carbonate salt concentrations impact osmotic functions in livestock. This flux will be shown as changes in the pH as well. So pH changes due to KH changes will affect livestock, but pH changes with no change in KH will not.
To this end, it is KH stability that matters much more in aquariums rather than pH stability. With the stability of the later (pH) being important only as an indication of stability of the former (KH).
Why all the emphasis on pH then?
It is more an effect of historical precedence than anything else; pH is easy to test for and understand, while testing for KH requires titration. With the improved understanding of today's science, we should shift our emphasis more onto paying attention to KH rather than pH because that is what ultimately affects livestock/plants.
So, one can ignore pH totally?
This does not mean that the importance of pH is totally negated, especially concerning specific sensitive livestock. However, in almost all of such cases, livestock that specifically require a certain pH range will also require a corresponding specific KH range; with the later taking precedence in importance. For example, you can keep African Cichlids in a CO2 injected planted tank (which drops pH close to 7), as long as you maintain high KH values in the water. Fish from acid peat swamps will do alright in a tank where the pH drops from 7 to 5.8 during CO2 injection as the low point in the cycle pH(5.8) is within range of their natural living conditions. However, alkaline water fishes may not take the dip into the pH 5.8 range well. It is not the swing, but the pH value itself being outside of the fish's natural range. The same alkaline water fish can survive a comparative pH swing from 8.5 to 7. It is important to differentiate the impact of a pH change vs the impact of an extreme pH value in 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 has the reputation of being a very sensitive fish - this batch have been thriving well 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 at pet shops is a testament to favorable tank conditions.
What pH works for fish?
A normal range of value where most commercially available livestock and plants can survive would be between pH 6.0 to 8.0. For most tanks, pH values do not fluctuate to a point of being harmful as long as the KH is kept stable. Livestock from regions of higher or lower pH levels can be more suited to the outliers on this range i.e. fish from acid peat swamps may do well in pH as low as 4.0, while many cichlids from alkaline water lakes are comfortable in higher pH ranges (8-9+) but do not favor acidic pH ranges.
If keeping fishes that are less tolerant to low pH; buffering the water to have higher KH levels will prevent pH levels from dropping too low due to CO2 injection. For most general community tanks, having 0.5 - 2dKH of alkalinity is more than adequate to prevent water from becoming overly acidic such that it affects livestock.
Why is pH so often quoted as such an important criteria if it is not really so?
This is a remnant of ancient 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 will affect pH readings - generally a stable pH means that KH is stable (which is what is important). A KH swing of more than 3 degrees in a short period of time can easily hurt sensitive livestock such as shrimp. And keeping KH stable is what is important. Nowadays we can measure KH, GH, TDS and pH separately, and our understanding of which variables impact 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.