That's basically who I'm referring to is the commonly available livebearers. Guppies, platies, mollies, swordtails, Endler's livebearers.Thank you. That one was taken in Venezuela. I was younger overthere and slimmer...
Well to be honest, there are also livebearer species which are better off in softer water. The hard water rule for livebearers is a generalization and based on the commercially available common livebearers.
I actually have a well on my property and I've tested the water. It's off the charts hardness. I could figure how much to mix with my tap water to get to the ideal hardness for any of those fish. But honestly, I'm good with soft water species for now. Maybe one day I'll use it for some African cichlids.