Sterilising unit discoloured

I have a tommee tippee steriliser which I clean religiously and de-scale at least once a week as recommended. The problem is the base plate is causing the water to discolour (brown) virtually straight away and is also causing the trays and bottles to discolour also. Has anyone else had this problem? I've emailed & called tommee tippee several times but as yet to get a response. HELP............