The lack of minerals that a very hard water fish like mollies suffer from when in soft water has a long term affect. They breed quite young, so surviving and breeding doesn't mean they're fine in the soft water, the damage is internal and invisible to us. But it makes them more vulnerable to various illnesses like the shimmies, and shortens their lifespan. Much better to choose fish that have evolved to live in your type of water.
For soft water fish in hard water, fish that evolved to retain the rare minerals they come across in their soft water habitat, they end up storing too many minerals from harder water and suffer internal damage as a result, usually kidney failure due to mineral build up. Shortens their lifespans too. A few generations of captive breeding can't over ride millions of years of evolution.
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