Can you please point to any scientific studies that show this? There's only one study that I came across and it only included a few species of fish. One of those species was Betta splendens. Ironically, that study found that Betta splendens fry had a higher survival rate in hard water. As I recall, the control group with the highest survival/weight gain was in water with a hardness exceeding 500 ppm. The only negative effect of hard water that I've ever come across is related to egg development.
You have correctly identified a large knowledge gap in science. To be honest, in general it's rather difficult to find research on many species in the hobby (except the very common ones like guppies, bettas, zebrafish, and a few tetras). There's even less research on the long-term effects of much of anything on ornamental fish. The thing is, there's very little reason to research these things. That is, there's almost no incentive. Ornamental fish are not particularly economically valuable compared to commercially farmed fish species and it's generally not monetarily viable to conduct years-long studies on any organism, let alone ornamental fish. There are few to no funding agencies that would fund this kind of research. When you do happen to find ornamental fish research regarding environmental factors, it's usually on shorter-term effects like acute stress or, as you mention, reproduction. Or, it's stuff that can be measured in a short time span, certain physiological processes like ion regulation, respiration, waste excretion, etc.
So, no, admittedly I have not been able to find a study that looks at long-term effects of hard water on soft water fish over the course of years. I will certainly keep looking and will get back to you if I find anything long-term. The vast majority of studies I found regarding water hardness effects on fish have to do with commercially farmed species like trout, salmon, and catfish, particularly in making sure there's enough calcium and other minerals in their diets. That's where the money is, so that's what the research is on. It's worth noting that just because a trend has not been quantified and published in a journal doesn't mean it doesn't exist. I'm all for finding academic sources. They're almost always the best sources of evidence we have. And it is good to question commonly held beliefs, but the fact remains that warnings of long-term effects of hard water on soft water fish have been present in the hobby for quite a while. Many common myths in the hobby are absolutely refuted by science, but until someone does the long-term research on the effects of water hardness, I have to go with what has been identified as a trend, even if anecdotally.
Now, this is pure speculation, but it is possible that soft water fish do benefit from higher dissolved mineral concentrations than what is found in their normal habitats. Soft water fish, especially in the Amazon, are well-adapted to quite acidic water with low amounts of dissolved mineral ions. Of course, they need these minerals/ions for various physiological processes. They also experience loss of ions (Na+, Cl-, K+) when exposed to very acidic conditions, although increased levels of calcium ions didn't affect loss rates, so it's questionable whether it's beneficial in that regard (Adalberto et al. 1998).
My speculation is as follows: it seems that fish in soft water have to "work harder" to get enough ions from the water, even if they're adapted to handle it. Water that's harder than their natural habitat may actually benefit them by relieving that constraint. However, I would expect that it's only beneficial to a point and that beyond that point, increased hardness wouldn't benefit them and could potentially have some detrimental effects.
So, I had a look at the paper you mentioned, because I was interested. I had to search for it myself, but I did find it. I noted some things that I'd like to comment on.
You are correct in pointing out that they found increased growth performance for the bettas with increasing water hardness. However, the researchers don't actually mention the period of time over which they measured growth. As per the methods, "growth performance" was determined by measuring adult fish at their sexual maturation. "Growth performance" appears distinct from measures of larval growth, since they're mentioned separately. In that case, what does "growth performance" actually mean? They bought fish from a local aquarium that were old enough to be sexed, so what does "at sexual maturation" mean in this context? How much time was there between initial and final measurements? The researchers don't say, at least not that I can tell. If "growth performance" was the growth of the fry up until sexual maturation, they needed to make that much more apparent because it is not clear.
Edit: Okay, I did find the length of time. It's 1.5 months. I rescind the clarity criticism.
Edit: double-checked the methods and the results table and yes, larval growth performance is distinct from adult growth performance. For the larvae/fry, they only measured length. For adults, they measured length and weight.
So, for the adult growth performance, it's still not clear how much time was between initial and final measurements. Also, betta fry growth performance (very bottom row) was actually LOWER at the two highest hardness values.
Ironically, that study found that Betta splendens fry had a higher survival rate in hard water.
No, they did not. Direct quote from the results: "The percentage larval survival of both species declined gradually with the increasing hardness level, reporting significant declines above 320 ppm (p < 0.0009)"
Additionally, it is mentioned that fish were fed twice a day. It's stated that the feeds were 5% of the fish's body weight, but it's also stated that they were fed
ad libitum, which in dietary studies usually means organisms were allowed to eat as much as they liked. So, it's not very clear exactly how much food these fish were getting. If they were fed
ad libitum, there would probably be significant growth regardless of water conditions. Even if they weren't fed truly
ad libitum, two feedings per day of 5% of their body weight is more than what's needed. University of Florida states that fish only need between 0.1-1.0% of their total body weight in food per day. With this in mind, the fish in this study probably would have grown regardless of water hardness.
Edit: I'm pulling back a little on this particular criticism because it's true that the adult growth was still significantly higher at the higher hardness levels, so it doesn't really matter that all the fish grew, it matters how much they grew, and the feedings were constant across treatments.
The control treatment, the lowest hardness level in the study, is 150ppm. That is
not soft water. Soft water is generally defined as 50-60ppm or lower. So, really, you can't use this study to compare betta growth performance between hard and soft water because the study only uses hard water at increasing levels of hardness.
Lastly, and most importantly, this study is NOT long-term. You cannot use this study as evidence that soft water fish do well, long-term, in hard water. At most, growth potential was measured
from hatching to sexual maturity over 1.5 months, which is nowhere near the full lifespan of the fish.
Link to the paper, for those interested in taking a look themselves:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7678082/
Adalberto et al. 1998 (ion loss in Amazonian fish):
https://www.journals.uchicago.edu/doi/10.1086/515893
University of Florida feeding guidelines:
https://edis.ifas.ufl.edu/publication/FA096
