WHAT IS IT, 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.
pH in natural water systems
pH swings from changes in CO2 concentrations are common in many natural bodies of water. CO2 builds up in natural bodies of water overnight due to organic decomposition. Once sunlight hits, plants begin draining the system of CO2. pH swings of up to 1 full point is common in many natural lakes/rivers; this usually takes place quite quickly between dawn and noon. Neither fish nor plants are affected by these changes.
Taken from Allen HL. 1972 Phytoplankton photosynthesis, micronutrient interactions in a soft water Vermont lake
pH fluctuations harmful? it depends on the cause
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.
pH fluctuations because of KH flux affects livestock; this is not connected to the change in acidity, but change in carbonate salt concentrations which impact osmotic functions. To this end, it is KH stability that matters much more in aquariums rather than pH stability. With the stability of the later being important only as an indication of stability of the former. 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 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.
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.
For hardy dwarf shrimp species such as these Bloody mary shrimp, daily pH fluctuations of more than 1 degree does absolutely nothing to prevent one from having a healthy, reproducing community. CRS is more sensitive; having exceeding high CO2 levels reduce brood sizes. Having a lower rate of CO2 injection, with attention to overall tank stability is importance for sensitive species.
The 2Hr Aquarist
Despite its fancy name, it just measures the total amount of Ca/Mg ions in the water (and other divalent cations). Calcium is present in most tap water. However, magnesium is often over-looked. If your tank has high GH, it can be that all of it is made out of Calcium ions, and no magnesium is presnt - in this case, magnesium still has to be dosed. Such data can often be found in local water reports.
Generally, hard tap water comes from limestone(CaCO3) regions - so its much more common to see high GH values with corresponding high KH values. However, these values do not have to match. It is possible to have high GH, low KH water. [Lots of Ca/Mg but no Carbonates] It is also possible to have low GH, high KH water. [Low amounts of Ca/Mg in water, but plenty of carbonates]
Plants and livestock are generally less sensitive to GH than KH. Plants that require softwater to grow well require low KH values, but not necessarily low GH values. These plants are sensitive to alkalinity, to be accurate, rather than General hardness. GH is important when keeping shrimp and shellfish.
Softwater tanks typically have GH as low as 2-3. While very hardwater tanks can have GH values in the 20s to 30s+. Plants can survive across a large range so GH should be tuned for livestock's sake generally.
GH in this tank was about 10-12, with KH values of around 8-11 due to the limestone used in the tank. I would classify such water parameters as moderately hard; even then 95%+ of available commercially traded aquatic plants would grow well in such conditions.
CARBONATE HARDNESS / ALKALINITY
It is a measure of the water's buffering capacity; the higher the KH, the higher the pH in absence of other chemicals in the water, and the more resistant the water is to downward fluctuations when an acid is added.
Pure water with 0 KH, and 0 other amounts of acid/bases will have a pH of 7. As KH increases, the pH will increase as well.
With regards to growing plants, between 1-2 dKH you can keep sensitive softwater species. Between 2-8 dKH you can keep 97% of all commercial aquatic plants in optimal condition. Between 8-12+ dKH you can probably grow 95% of species well, but some will be sub-optimal. Above 18 dKH or so, more plant growth issues start arising - at this level, hardy plants such as Java fern, Anubias, Vals, certain Swords and Crypts will still grow well, but many other species will stunt.
Many more sensitive freshwater fish species may have the same preference for lower or higher KH ranges, although the majority of commercially bred common ornamental fish function well through a large range (i.e. 1 to 10dkh). If you are intending to breed specific species, checking up on their requirements before hand is important.
Your tank needs to take into consideration both the requirements of both livestock and plants.
KH has a significant impact on livestock osmoregulation, and should not be changed rapidly for sensitive species such as ornamental dwarf shrimps. A 3dKH swing in KH value is significant enough to stress sensitive livestock. When purchasing such livestock, it makes sense to get them from a dealer with similar water parameters to your own.
If you need to prioritize one parameter to keep stable; keep the KH stable. Managing water parameters well allows one to keep sensitive livestock in a planted tank.
TOTAL DISSOLVED SOLIDS
TDS refers to Total dissolved solids; this include all dissolved organic & inorganic substances in the water.
100pm of Calcium in water is relatively harmless, 5ppm of copper will kill most aquatic life. Dosing fertilizers in a planted tank will naturally raise the TDS value; in their simple elemental forms most fertilizers are non-toxic to livestock unless over-dosed greatly.
Interpreting TDS values
Very high levels of TDS can induce salt stress in plants/livestock. However, unless you know what the TDS value comprises of (what elements are dissolved in the water to give rise to the TDS value), it is hard to say at what PPM it becomes harmful. Freshwater by definition has less than 1500 ppm TDS, while seawater is generally more than 5000ppm. By no means does this mean that if you have water below 1500ppm it is fine ! The advantage of having a lower TDS reading is that generally it means you have less unknowns in the water column.
However, if one tests tap water regularly, and find that it's TDS value fluctuates a wide range. (more than 50ppm) This is a good indicator that the tap water is unstable, and it would be wise to test the other values as well to make sure they are not too far out of range (KH, pH etc).
TDS is thus useful as a relative test.
This relative test can also be applied when adding new hardscape or other materials to the tank. If you soak a bunch of stones in a bucket of water, and the TDS rises over time, it means that some part of the rock is dissolving. Totally inert rock will not change the TDS value.
Tanks that use limestone like mine above can see high TDS values at 400+ppm without issues for neocardina shrimp or plants. Other more sensitive species may prefer a more specific (usually lower) range.
This relative test can also be applied to organic waste. Organic waste also raises TDS; unlike what most aquarists think, organic waste doesn't transform magically into just ammonia. Breakdown of organic waste goes through intermediate stages that releases carbohydrates, lipids, proteins, nucleic acids into the water column.
Having a good bacterial cycle quickly decomposes these into simpler elements. High levels of undigested organic waste trigger algae blooms. Organic waste comes from livestock and deteriorating plants (either plants not growing well, or old growth), an easy way to get rid of this is by doing regular water changes.
If you have a matured tank and wonder if organic waste is building up in the water column - whether water changes are necessary; measure the rate of TDS increase over time. Even as ammonia, nitrite/nitrate readings remain at 0, tanks that build up dissolved organic waste over time will see rising TDS values.
Targeting a minimum value?
Many natural planted biotopes have extremely low TDS ranges; 25ppm and below. Plants are very good at scavenging low levels of nutrients. Plants can still grow well by taking in nutrients from the substrate zone even if the water column is very lean. The important thing is to make sure all nutrients that plants requires are present rather than target a certain TDS range in a particular tank.
Brian Jones's tank here runs well on a TDS of just 30ppm.