Heliamphoras do not like to be wet all the time. Misting systems can be cool to watch, but IMHO they are totally useless, and they bring a lot of problems, including mold and other fungus Sp.
Insitu, helis are often wiped by intense rain storms, but the ventilation is also intense so they get dry very fast.
I think your plant struggle for this reason. I dont think your water type is the problem here. Just stop your misting system, provide a very humid setup with a slow but non-stop air flow, no standing water, a airy soil mix, good lighting and everything will be allright. Also, no need to drop the temp at night. If your day temp is around 21-22C, nights at 19-20C is good for most species/hybrids. However, colder nights will help with coloration of the pitchers
Hope that help !
Francois