what is carbon used for then?
non-planted tanks with high bioloads? to remove all the amonia from the
tank?

No, carbon won't help with ammonia.

Carbon helps with water clarity, and absorbs things like heavy metals. It can
also help if the tank smells bad.

But a well-run tank really doesn't need carbon, planted or not. I use it for
specific, temporary purposes only: removing medication after treatment is over,
clearing the water if you've just put in dark piece of driftwood that's
staining the water, etc.

I haven't used carbon in my filters on a regular basis for years.


Leigh

http://www.fortunecity.com/lavender/halloween/881/