Okay, but is it in any way harmful if I use it?

Some people worry that it could remove nutrients from the water that plants
need. Others say that after a few days, carbon actually starts releasing
whatever toxins it has absorbed.

However, a little carbon probably won't hurt. The amount of carbon that comes
with most filters (like the Whisper pads, or the cartridges that come with
UGFs) isn't enough to make a difference, one way or the other.

Those pads and carbon inserts cost money, though. I prefer reusable media.
Sponge inserts, bio-balls, etc. You can just rinse and reuse them for months
or years. No need to buy new media all the time.

Leigh

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