Hmm, I posted a response but apparently it didn't show up...what was I going to say? @_@
If the case is that it only turns mucky when the water company flushes it, then there is something wrong on their end (not just the dummy contractor, haha). You should definitely call up and complain, or write a letter. The water company cannot honestly expect people to live in such conditions, let alone aquatic pets.
In any case, I'm glad that a senior member helped you out, and I wish you and your fish best of luck in health!