I had PFR in 0 KH and they survived but I got no breeding. Don't know if that's part of the reason. The pH was low too, around 6.4. They did live though for about 3 months before I moved them to their current tap water tank which has water pH 7.6 and KH 5. I'm getting better colors and lots of breeding now.
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Someone else might know for sure but I believe the ph is more important. When I lowered the ph in my tank the breeding stopped for a while. They're breeding again but not as much as previously. My crystals are breeding like rabbits though which is why I altered the tank