Its Duckweed.

> Date: Thu, 18 Apr 1996 08:29:00 -0500 (CDT)
> From: Wehde, Mark B. <MBW01 at msmail2_mayo.edu>
> To: owner-aquatic-plants <owner-aquatic-plants at actwin_com>
> Subject: what is this?
> After planting my last order from Delaware Aquatics, I noticed about 100   
> small ( about 1/8" ) green 4 leaf clover shaped leaves.  They float on   
> the top of the tank and have since ( 2 weeks ) put down about 1" roots.   
>  I think they were in the package with some hygrophylia polysperma   
> "tropical sunset", but could of been with the bacopa or red ludwigia.   
>  Does anybody have any ideas what these are?
> Thanks for any help
> Mark

This sounds very much like Duckweed. It is a prolific floating plant which
will probably need thinning out regularly, it is usefull for extracting
nitrates from your water.