KH <= GH

[sciencefiction]

Some "black-water" fish can only be maintained in soft acid water. This may relate to their inability to survive in environments with a high bacterial load, because the acidic, oligotrophic habitats have a very limited, and specialised, microbiota.

I think you're right on the spot, that certain fish are just not immune to a lot of micro-organisms that flourish in hard water, and it is where the problem comes from, and not the hardness of the water. But a trigger is needed first to further weaken the fish's immune system.....Improperly set up tank, be it lack of enough oxygenation, insufficient filtration, blocked filters, too small of a tank, too many fish, aggressive fish kept with peaceful fish, insufficient water changes, cheap food filled with fillers and no proper nutrition, too much food, too little food, etc...any of these factors will lead to fish diseases way faster than the hardness of one's water..
What we consider sensitive fish, are fish that are not naturally immune to common fish micro-organisms and bacteria and which are subjected to additional negative factors to further weaken their immune system. If the tank is not right, even in their ideal water the fish will still succumb to diseases...

I am not familiar with the physiological processes happening when a black-water fish is kept in hard water. I can only say from my own experience of keeping 3 types of black-water fish in my hard water that there are a lot more important factors to their well being than how hard the water is.....I'd say people may be finding excuses in the hardness of their water when they fail to keep fish alive....I know you're familiar with my fish arsenal but I keep clown loaches, kuhli loaches and harlequin rasboras...They don't show any ill effects and neither of them have fallen sick. Oldest are 6-7 years old now(clown loaches and kuhli loaches) and healthy. I hope they remain so for a long long time....However, it will not be my water that would kill them prematurely, but the lack of fresh water....or the lack of proper care on my part, or an accident of some sort.

I think one should keep the fish that inspire them to be a fish keeper, and take great care of them with clean water and suitable setup, whether soft of hard water...is not that relevant for majority of fish.

 

[sciencefiction]

There is discussion wether the 1 point drop will be correlated to the same CO2 level in harder and softer water.

It will not correlate to the same....Having hard water means you need a lot more [H+] to negate the existing [OH-] normally present in high levels in harder water. Thus 1 point drop in hard water is a lot more CO2 injection than the same in soft water. However, if one's KH is low, regardless of whether the water is soft or hard(meaning GH), it will take a lot less CO2 injection to move the pH 1 point down. The main point is, all this doesn't matter as CO2 injection doesn't alter the properties of the water meaningful to fish. It is only dangerous on its own, as CO2 is toxic to fish...

 

[zozo]

Yes, because when you add soda bicarbonate, you only add [HCO3-] and [OH-], but not Ca2+ or Mg2+ , so dGH remains unchanged.

But even tho it is only measuring alkalinity it is expressed in °DH, thus it never the less is adding hardness.. And if there is nothing else in it, than this obviously is the Total Hardness.. As a trivia interpreting the GH as absolute Total Hardness no matter the elements than @Edward Shave and his source is 100% correct with °DKH can never be greater than °DGH.

And since we have °e (°clark), °fH,°rH, °aH (in ppm) i can understand the general envi towards °DH writen completely in capital letters triggering a trivial discusion about "What °tf is what???" And so on the scientific world got completely fed up with this triviality and decided since 2007 lets make it world wide mmol/l and done with it!?

Publicaly you can not change this drasticaly, it doesn't sell, only will result in enormous amount of °tf phone calls to the water company, since the majority is still old fashion generation.. Thus let them ponder a while longer with °DH to let them know what to expect from their coffee machines lifespan.

If the same question is asked 20 years from now all the kids will ask "What era are you from? Who is still using °DGH?..

 

[sciencefiction]

But even tho it is only measuring alkalinity it is expressed in °DH, thus it never the less is adding hardness..

CaCO3 + CO2 + H2O → 2 HCO3- + Ca2+

Well, the above reaction is adding both hardness and alkalinity