[juan]

I would think that somewhere you do have charcoal in the system. Or who ever cleaned your system used charcoal. If your fresh water tank has a drain, drain some out and see what color it is. If it's clear you have a filter that needs to run a little longer to flush out the charcoal residue. If the water in your tank is gray you need to go back to whoever cleaned your system, they probably used a system that included charcoal filters.