My apologies for the belated reply. Been at the hospital for a few days.
Well, a lot of stores also use the name "Poecilia wingei" for endler hybrids. Officially that would be incorrect to use that name. But a lot of stores think it's more interesting and also a selling point to use "Poecilia wingei".
So, I don't know wether you're having pure ones or hybrids. The parameters you've mentioned should be higher. It's already been mentioned overhere by other how to increase the hardness.
I hope you feel better!
Thank you for your advice, I'll work on increasing it, whats a good way to keep it stable?