[Matt Nace]

What size tank?

Are they regular Comets?
Comets are really messy.

 The junk in the substrate has to be vaccumed out as well.
 If this is a small tank, you may need to do an additional water change inbetween your regular ones.

I suggest you test for Ammonia, Nitites, and nitrates.

My mother-in-law had a comet in her 55 gallon, which was so cute at first. It ate..and ate...and ate...then it was super big and fat. It caused an ammonia problem, and the water clouded. The end result was me takin it to the LFS.

If these are not comets, but fancy tails, then I would guess you were not getting the detrius and decaying food out of the gravel.....which you took out.

I dont use any carbon or chemicals in my tank, except to dechlorinate the water going back in.