"Back when" on the American frontier where farmers were too often malnourished by a diet of mostly corn and pork products, women would lose most of their teeth after several children. So that her baby's bones were well formed, her body would rob her teeth (and perhaps her bones) of calcium.

I suppose that could happen with guppies. The flake diets are pretty well balanced and formulated though. Certainly most Mid-Westeners and people from the Great Plains and West Coast, with very rare exceptions like parts of PeterW's San Francisco, shouldn't have to worry about there being enough calcium, magnesium and potassium in their water. I wonder if guppies were raised in the Far East and they raised the mineral level only with several handfuls of what is essentially table salt, then if the potassium needed for a number of body functions and the calcium for the bones may be deficient. Even using a marine salt mix, there are other minerals than sodium chloride.

See also spinal deformity (guppy scoliosis) in Immediate Help.

It is funny how that works. Neon tetras from the Amazon or (the majority on the market) from the Far East naturally thrive in the relatively low mineral levels of those rain forest areas. If American aquarists buy those neons when they are young and raise them in water of moderate to low mineral levels, they very well will be able to spawn them. If the neons are raised in pretty hard water, they will not be able to spawn.

Some rain forest egglayers like killies will eggs which never successfully hatch. The mineral level may have to be dropped to under 200 PPM hardness to get them to even lay eggs. If the embryos die within the eggs, it is suggested that the water be  gradually cut (with rain water, RO water ....) until the hardness is 25 to 50% less.

Most aquarium fish are amazingly hearty (or they wouldn't be in the trade). Getting egg layers to successfully spawn and grow up may be a different game.

Some otherwise fine potential aquarium fishes are not imported for the mass market. A few of those are brought in by and for specialists who know how to play with the water. Some times hobbyists save up and consulting with academics, other specialists and (by all mean) the government of that place, they will seek them out. That could run thousands of dollars for a few pairs of something rare (by hobbyist standards).

There are now companies such as Margareta Tours http://www.amazon-ecotours.com/expeditions.htm
which run all sorts of tours. Among them are fish collecting trips. One of those was written up this summer in a couple TFH articles by Randy Carey. A person always wonders. The fish are sometimes what we already have in the hobby.