Keeping Fish At Extremes Or Outside Parameters

I'm aware that this may be a slightly controversial topic, and I'm trying to get some opinions.
It can sometimes be confusing when the information across several sites is slightly different and I've been researching information on the internet regarding keeping fish in harder water. The conclusions that I have come to are that the water parameters stated are related to their natural environments, and that farmed fish for the aquatics trade can sometimes be more tolerant of wider parameters than those of wild fish. Some sites stated that a problem with harder water is that the fish won't breed.
I have hard/very hard water of 17dH German, and try to only purchase fish suitable for my water. However, I already own a bristlenose plec and a blue-eyed ancistrus, which usually have an upper range of 15dH.
I spoke to some staff at a LFS today and they agreed that farmed fish can be more tolerant of harder water, and that they use tap water for most of their tanks, with the exceptions of things like apistogramma where they mix tap and RO. I am also aware that fish are only at the aquatics store temprorarily, rather than being exposed to harder water for their entire life, which I'm sure is quite different. They also said that they keep fish at home in the same conditions.
So, my question is basically how hard is too hard?
Is it acceptable to keep fish that are tolerant of up to 15dH in the wild, but are farmed for the industry, in water that is 17dH?
Is is acceptable to keep fish that are tolerant of lower levels (e.g. down to 12dH) as long as they have been bred for the industry and have been kept in harder water?

I am prepared for a rousing chorus of "Donna, don't be so irresponsible as to keep fish outside their stated range just because you really like them and want to keep them", or similar, but would prefer for any "talking to" not to be too much harsher than that, if possible.

Thanks.

I am another 'within reason' advocate. I would not keep some of the more delicate Amazonian fish in your water, neither would I keep Rift Lake cichlids in mine. And no wild caught soft water fish in my tank either because although my hardness is just 5 German deg, my pH is 7.5. But I have successfully kept endlers at 5 deg, and they are hard water fish.

Provided you do your research and know which species must have a certain type of water, and know if the species you want will be tank bred or wild caught (shops won't know, they buy them of the wholesaler who buys them off the importer) I see no reason to go slightly beyond a fish's preferred range.


And the main reason soft water fish won't breed in hard water? The egg outer layer absorbs calcium and the egg either doesn't develop properly or the fish can't get out.

These days the vast majority of fish are commercially bred. Nearly all fish in shops are juveniles, if fish were wild caught they would be mainly adults.

@Richard W I hadn't thought of that, and it makes perfect sense to me.

@ColinB this was one of the articles that I found
http://www.seriouslyfish.com/whaddaya-mean-too-hot

@Sue the thought of fish not being able to get out of the eggs made me feel a bit   

I think the "within reason" seems to be a sensible approach, and keeping fish that are farmed, and not too far outside their stated/natural parameter range, should avoid too much trouble.

However, I have also found a few fish that are fine up to 30dH, and can only assume that they come with crampons and other mountaineering gear rather than fins! They should have no problems with my 17dH water.

@ColinB this was one of the articles that I found
http://www.seriouslyfish.com/whaddaya-mean-too-hot

This was the article thet prompted me to ditch the heater.

Not thought about it not being wasted heat as such during winter... makes me feel a lot better about things... though I did just get £500 back from my energy supplier as I've been overpaying since the installation of our new boiler nearly 2 years ago so heating my 64litre will be almost unnoticeable in comparison 

I set up all nine of my tanks within a 6 month period. Four of them are in an unheated room where the temperature can drop to 10 degrees in winter. My electricity bill rose by £300 a year. I keep the tanks at about 23 degrees and can never understand why so many people insist that they should be at 26. During the colder part of the year, the tanks in the unheated room are covered with old blankets at night.

However, I'm sure that the lights use more electricity than the heaters. Fluorescent tubes aren't that efficient which is why I intend to convert all of mine to LED d.i.y. strip lighting as described by various people here. Lights are always on during the day, whereas heaters are probably on more at night when I have economy 7 electricity at a third of the day time price.