The water can be soft to hard. No preference to pH. Likely were sick and not well ropes to begin with.If i remember correctly don't ropefish prefer a slightly acidic ph too. They've been in three sets of parameters in the space of a short time. If those ph parameters were all over the place could you be looking at some kind of mild to severe ph shock which may have left them vulnerable. It really could be anything though. I hope your remaining three pull through, good luck.