I had a similar problem with my tropical tank a few months ago. Luckily, I caught it in time and no fish died. I simply cleaned the filter, threw out the carbon (in all my other filters too!), did a 20% water change, added a bit of salt, let it filter for 24hrs then put the fish back in and it was fine.
I've stopped using carbon as a result, and add a bit of salt every so often, which seems to help all the fish too.