As far as I know fish do not have a KH requirement. I have only seen two sites that even mention it. One is the site linked by Ice and the other is LiveAquaria.com. I actually called them and asked if they could provide me with links to the science that supported their belief that fish have HK requirements, they could not. Based on that and other information I found on their site, I refuse to buy fish from them ever. That is my opinion.
In trying to find research indicating the are alkalinity requirements for fish, I cannot do so. Alkalinity is important in terms of the overall "quality of water," but it is also highly variable. Co2 plays a major role.
Alkalinity is also referred to as temporary hardness. So many factors can affect alkalinity whereas hardness as GH tends to remain more stable. But the fact that temporary means just that, it can change. Heat water and the kH will drop and that likely will result in a drop of pH.
But let's fall back on a little common sense. Visit an assortment of fish sites which list the parameters for various species of fish. The look at what measurements they offer. You will almost always see: temperature ranges, pH range and hardness range (as GH or conductivity/TDS). What you will almost never see is a required range for Alkalinity aka KH. If this were a major factor, surely we would see it everywhere?
This is not top say that alkalinity doesn't matter, it does. The reason is that alkalinity does contribute to conductivity/TDS but, more importantly, it is used up by various processes in a tank. However, alkalinity is what keeps pH steady at any given level. A decrease in alkalinity is normally accompanied by a drop in pH and vice versa. Here is a decent piece that may help you make more sense of KH nad GH and total hardness and total alkalinity:
https://edis.ifas.ufl.edu/ss540
Here is how that link describes Alkalinity and Hardness. The thing that should stick out to you I have highlighted in yellow at the end of each's description:
Alkalinity
Alkalinity is a measure of the acid-neutralizing capacity of water. It is an aggregate measure of the sum of all titratable bases in the sample. Alkalinity in most natural waters is due to the presence of carbonate (CO3=), bicarbonate (HCO3-), and hydroxyl (OH-) anions. However, borates, phosphates, silicates, and other bases also contribute to alkalinity if present. This property is important when determining the suitability of water for irrigation and/or mixing some pesticides and when interpreting and controlling wastewater treatment processes. Alkalinity is usually reported as equivalents of calcium carbonate (CaCO3).
Hardness
Hardness is most commonly associated with the ability of water to precipitate soap. As hardness increases, more soap is needed to achieve the same level of cleaning due to the interactions of the hardness ions with the soap. Chemically, hardness is often defined as the sum of polyvalent cation concentrations dissolved in the water. The most common polyvalent cations in fresh water are calcium (Ca++) and magnesium (Mg++).
Hardness is usually divided into two categories: carbonate hardness and noncarbonate hardness. Carbonate hardness is usually due to the presence of bicarbonate [Ca(HCO3)2 and Mg(HCO3)2] and carbonate (CaCO3 and MgCO3) salts. Noncarbonate hardness is contributed by salts such as calcium chloride (CaCl2), magnesium sulfate (MgSO4), and magnesium chloride (MgCl2). Total hardness equals the sum of carbonate and noncarbonate hardness. In addition to Ca++ and Mg++, iron (Fe++), strontium (Sr++), and manganese (Mn++) may also contribute to hardness (APHA et al. 1998). However, the contribution of these ions is usually negligible.
Hardness is usually reported as equivalents of calcium carbonate (CaCO3) and is generally classified as soft, moderately hard, hard, and very hard. It is best to report results as the actual equivalents of CaCO3 because the inclusive classification limits for each category may differ between users of the information.
What you should take from the above is the word
equivalents. What this means is what is being measured is not just CaCO3, but is expressed as if it were. So two water samples with different things in them may still give you the same KH reading. The problem is these two samples would not support the same fish.
What you should further take away from the above is that a whole lot of things are included in hardness but not in alkalinity. However, total hardness contains both GH and KH as they overlap. Calcium carbonate breaks down in water. The calcium contributes to hardness (GH) and the carbonates to Alkalinity (KH). Moreover, there are more things in water which contribute to conductivity/TDS but which are not measured by either GH or KH.
Finally, none of the measurement of GH, KH, Total hardness, conductivity/TDS report what it is exactly that is contributing to any of these measures in any water sample tested. This requires much more in depth equipment that none of us will ever have nor will any fish/pet store where you might have your water tested. This stuff requires a real lab and very pricey equipment.
My advice is to ignore KH for fish as a requirement as you would for temperature, pH and Total Hardness or conductivity/TDS.
I did find this study which investigated alkalinity:
John Colt, Eric Kroeger,
Impact of aeration and alkalinity on the water quality and product quality of transported tilapia—A simulation study,
Aquacultural Engineering,
Volume 55,
2013,
Pages 46-58,
ISSN 0144-8609,
.
(
https://www.sciencedirect.com/science/article/pii/S0144860913000253)
Abstract:
The effects of aeration and alkalinity on water quality and product quality of Nile tilapia (Oreochrmis niloticus) were determined for simulated commercial hauling conditions. Three types of aeration were tested: pure oxygen aeration with a fine bubble diffuser (Oxygen), air aeration with medium bubble diffusers (Air), and a combination of both pure oxygen aeration with a fine bubble diffuser and air aeration with a medium bubble diffuser (Mixed). Simulated transport hauls were conducted at two initial alkalinities: 1.74±0.11meq/L (Low) and 8.84±0.55meq/L (High).
The Air treatments resulted in the lowest carbon dioxide concentration, and the highest pH and un-ionized ammonia concentrations. At high alkalinities, the Air treatments were unable to maintain adequate dissolved oxygen levels. The Mixed treatment resulted in reduced carbon dioxide and dissolved oxygen concentrations. The Oxygen treatment resulted the highest dissolved oxygen, highest carbon dioxide, and lowest pH and un-ionized ammonia. Un-ionized ammonia concentrations were higher with the High Alkalinity treatments because of higher pH. Significant mortality was observed in the Air treatments in both the Low- and High-Alkalinity treatments. Mortality in the Oxygen and Mixed treatments for both low and high alkalinities were comparable to that observed in commercial tilapia transport using fine bubble diffusers and pure oxygen.
These results indicate that mortality due directly to hauling water quality will not be increased at high alkalinity, if pure oxygen aeration is used. The potential effects of water quality during hauling on survival and product quality may be less than the impact from (a) physical damage from loading and un-loading and (b) physiological problems resulting from pH and temperature shock during the transfer from the hauling tanks to retail holding systems, especially for fish of reduced fitness.
