I don't have the fountain, but I do have a turtle tank. I use carbon in my filter. I have had many times where I have cleaned the tank. It seems like I rinse the stuff for an hour and it is still black!!!
I'll be interested to see some suggestions. I soaked my carbon in water for a while and then rinsed it some more. It seemed to help, but that could be because I was rinsing it for along time before that...LOL