toilets for the fishes

[Zamshed <zamshed at abnetbd_com>]

Hello Everybody,
Few days back, I added 2 stones stuffed with some kind of locally collected 
Willow Moss in my 50 gal aquarium.  They are dark green and hairy.  While 
observing those I found all of the fishes visiting the moss, few are even 
taking rest on moss (black moly, rainbow shark, tetra and zebra are among 
those), flapping their fins and returning one after another.  I asked 
myself what they are doing on the moss?  To my surprise, I discovered that 
the fishes are coming to discharge their waste on the moss.  The waste is 
decomposing very fast and fresh matters are added every time.  My family 
members are really enjoying it and we are at last happy to make  toilets 
for the fishes.  Is it normal or what may be the cause of it?

Best Regards,
Zamshed / BANGLADESH