The question was asked of me by Nemo2182 (OP) in chat, how GH affects fish. I do not like chat, so I will answer in this thread.
Each species of freshwater fish has evolved over thousands of years to function in a very specific environment. By "environment" here we are meaning everything that affects the fish's life, from the water parameters to the habitat properties (substrate, wood, rock, plants, leaves...), water flow, numbers of the species when shoaling fish are concerned, other species in the watercourse, light from the sun. All of these factors determine how the fish will function; how its physiology operates in response, how the internal processes work that maintain for example the pH of its blood the same as the water it lives in, the immune system, and so forth. Every aspect of the fish's life is governed by the environment. This is uniquely more important for fish than any land vertebrate with the exception of amphibians...and here it is water again that determines it.
The GH is the measure of dissolved calcium (primarily) in the water. Fish that have evolved in moderately hard water need this calcium. Fish that have evolved in soft water do not. Their physiology determines how they respond to whichever.
Water is continually entering the fish via osmosis through every cell, and at the gills. This is 24/7. Substances dissolved in the water enter the fish, into the bloodstream. As this water passes through the kidneys, the process of osmoregulation determines how the water is processed. Mineral salts like the calcium are extracted by the kidneys. In hard water species these mineral salts are essential to the proper functioning of the internal processes that work continually to maintain the fish's equilibrium; without these minerals the fish slowly weakens and dies. In fishes evolved in soft water, the salts are "foreign substances" that the fish does not have the ability to deal with, simply because it was never designed to do so. Calcium builds up, blocking the kidneys, and the fish dies.
The fish's internal biological processes are governed by the environmental factors mentioned above. The fish must for example regulate the pH of its blood to equal that of the water in which it lives, ensure the tissues are fed, the immune system functions, etc. The fish's physiology depends upon factors determined by the environment. As soon as these factors are changed from what the fish is programmed and designed to use, it creates stress, but even more significantly, the fish must expend considerable energy attempting to "right" what is "wrong." And generally it cannot do this, at least not for long, or depending upon the degree of difference. This means the fish slowly weakens. Usually the severe failure of the immune system causes the fish to die from some disease or issue that it normally should have easily dealt with, but could not because of its weakened state. In all cases, should the fish somehow miraculously succeed in avoiding disease, it has been weakened to such a degree that it can no longer support life processes and it dies, prematurely. There are no external signs of any of this, until the fish dies. Necropsy (autopsy in animals) can determine exactly which process failed.
The author of an article in TFH a few years back had a good analogy. The fish living in the environment for which it is designed by evolution is comparable to a car traveling along at say 30km per hour on a level road. Everything is working well, and the car reaches its end goal. But if it encounters a "foreign obstacle" like a steep hill, it must immediately kick into overdrive, and use considerably more energy to maintain the same level of speed. Thus the fish encountering any environmental factor that is outside its preference has to kick into high gear in an attempt to maintain the essential bodily functions of life. Most fish, depending upon the degree and other factors like size (larger fish species have better ability than smaller fish species), can maintain this heightened state for a brief period, to get them through some immediate problem, but not for long. The fish weakens the longer it is forced into this situation, until it simply can no longer cope.
Again, there is no external sign of this. Many will say "my fish are happy," ...but the fact is, they are anything but happy. And this is a completely insane comment...unless your fish can talk to you, there is no way any of us can ever say this. We can and must understand that when we do not provide a species with what it is programmed to expect, they will valiantly make an effort to make the best of what we have forced upon them, struggling to maintain a normal and essential set of processes, but we have thrown a wrench into the works and it does always have consequences.
I would not want to hurt them. So is it worth me mixing half distilled with half my current hard water? Will I need to add anything back that way? I want what's best for the fish
