I'm torn. I kinda love your attitude towards fishkeeping, and agree with a lot of points, and that often, we overthink things, or get too obsessive about rule following, and risk taking a lot of fun out of the hobby, or ruining it for ourselves and perhaps our fish. I really do value your input on the forum! You have a lot to contribute, I really mean that, I enjoy your posts and find them helpful.
I'm also still too new to the hobby myself to really have a firm opinion, so I'm sitting on this here fence, thank you very much!
On the other hand, the scientist side of me sees the sense in keeping fish according the habitat they evolved to live in. Now, there's often a large amount of leeway for certain parameters like pH and GH, with different species being much more adaptable than others (often because they have a large range in the wild, being found in areas with quite different water chemistry). And a lot of species like livebearers have been farmed and bred in captivity for so long, that they may have adapted more than other species may have.
But I think it's still worth knowing about and considering what your source water's GH is, and what species tend to prefer that hardness. Mollies love super hard water, and if someone keeps losing their mollies, that they tend to wither away then go belly up, finding out they have very soft water then gives them the option to look for other, soft water species, or add minerals to their water so they can try again with mollies.
Excuse the poor paraphrasing, but it's too late at night for me to go hunting for the source right now, but essentially, hardwater fish like mollies evolved to live in water with a heavy mineral content, so they pass a lot of those minerals, but get exactly what they need. Soft water fish evolved to hang on to certain minerals when they came across them, since there isn't much in the water they evolved to live in. When kept in hard water, they retain more than needed, often leading to blockages in their kidneys, leading to an early death.
I didn't know about this when I first got otocinclus, and added them to my hard, 253 ppm water aquarium. They seemed very healthy and thrived, even appeared to spawn once. But I lost them one by one, suddenly, without any other apparent cause, after a year or so. They just died off one by one. I strongly suspect blockages by being kept in harder water than they've evolved to handle.
Now one of my tanks I use rainwater mixed with my tap water to make it softer, for the remaining otos and pgymy cories. I'll get more otos for this softer water tank at some point, and it remains to be seen if this gives them a longer lifespan. Something like internal damage is impossible for us to see, so even if fish look normal, breed etc in the "wrong" water, it doesn't mean that internal damage isn't being done, you know? The fish don't have a choice about what water they're in, and like any creature, they'll do their best to survive -even if they aren't thriving.
Just my two cents! It's gotta be up to the individual aquarist whether it's worth it to them to make adjustments, or whether they want to risk it, or even if they believe it. Just personally, I think at least knowing what the species you're keeping evolved to live in, and trying to replicate that as best as possible, is just good animal husbandry, and might save some heartache down the line.
Scratch being too lazy to find the souce, Neal Monks is a pretty trusted fish authority/scientist, I certainly trust his opinions.
Quote from the article:
"So, that's why water is hard; what does that mean for the aquarium? Besides raising the pH, hardness salts affect the animals and plants in the aquarium in a number of other ways, too. For some small fish adapted to soft water conditions, the minerals in hard water are thought to cause blockages in some of the organs. Dissection of neon and cardinal tetras has revealed damaged kidneys in specimens kept in hard water aquaria. That said, the majority of soft water fish generally do tolerably well in hard water aquaria. The problem doesn't tend to be that the fish die prematurely, though some do, but rather that it becomes impossible to get the fish to spawn or to raise the fry. Some species simply won't breed at all in hard water, while others, like Kribensis, will spawn, but the resulting fry invariably show a preponderance of a single sex within the brood."
