To What Degree Can Fish Become Acclimatised To Hard/soft Water?

afternoon fishy friends!
I have  a query relating to the degree of hardness/softness of water.
I have a delivery man who regularly delivers work parcels to me and who, on spotting my tank when he last made a delivery, told me that he breeds guppies and has been doing so for 15 years. Now, I do quite fancy having some guppies in my 180 litre juwel Rio, but I've always been put off by the fact that guppies should be in hard water and all of the water in Scotland, where I live, is very soft.

So, my question is: if I wanted to get some guppies from this particular breeder, would they be OK in my tank? He lives in the same town as me, so his water is identical (he doesn't use RO water or change his water in any way) and he's happy for me to go along to see his set up and choose some fish if I like them, and his guppies have been bred in this water for 15 years.  This guy also supplies guppies to the local pet shop, but I don't like the way the pet shop owner operates, so would never buy fish from there. But I would be interested in buying fish from the breeder.

What's the general consensus? Would it be Ok to buy guppies from this breeder?

All common fish, including guppies, have been bred for hundreds or thousands of generations in captivity, in a wide range of water conditions. Those which can't tolerate a wide range of conditions die, they don't get to breed. Those who do survive pass on their tolerance to their offspring and so over the generations each species becomes more and more tolerant to different conditions. It's a form of natural selection, except that it isn't exactly natural, being under human agency. For these low cost fish, it just wouldn't be economical to supply special water conditions, they have to take what they are given and sink or swim, literally.

Unfortunately, information that was true for fish from the wild is still passed on by books,, web sites etc., even when it is well out of date. Cardinal and Rummy nose tetras, for example, can live perfectly happily in hard water, the ones I inherited with a used tank are at least 4 years old and have been in hard water all their lives, still healthy and lively. The same will be true in reverse of guppies, I am sure they will be fine in virtually any water. It really doesn't matter where they come from. This is not acclimatisation though, you can't take an individual specialist soft water fish and slowly change it to tolerate hard water, it's what's in the genes that counts.

This doesn't apply to specialist fish, such as discus or Malawi cichlids since these are usually bred by specialists, rather than mass market fish farms, and are always kept in the "correct" conditions. There is therefore no selective pressure on them.

There's no guarantee that your guppies will do well, they are so inbred and weak that they may die anyway. Personally I think there is a moral issue with guppies, as with bettas, that everybody only wants males. So what happens to all the unwanted females? I suspect most are euthanized or fed to other fish. Just another reason for me not to keep guppies. This issue may work the other way e.g those fish where people are advised to keep only one male with 2 or 3 females, inevitably there will be an unwanted surplus of males. Much like the situation in the egg industry where millions of day old male chicks are gassed every year, a few go to feed reptiles but most end up in landfill.