Mysterious tiny plant (and black algae)

I thought that I had dust in my biggest tank (280 liters or 75g, open
top, very little surface movement because both filters are nearly
clogged), but as I looked closer, I noticed that is was the smallest
floating plant I had ever seen - and flowering!

The mysterious plant looks most like duckweed (Lemna minor), but
it is much much smaller, the diameter of a leaf is only about 0.5 mm
(that's about 1/50 inches).  There are always two or three leaves
connected together, like duckweed, and the color is very light, more  
yellow than green.  There are two white flowers in every little plant, 
and they are about same size as a leaf.  I can't see any root structure 
or anymore details because the plant is so small and my eyes are not as 
sharp as they have been.

Any ideas what it might be?  It does not look like Wolffia arrhiza
(in Baensch vol 2), but perhaps another Wolffia?  They are supposed to
be the smallest flowering plants in the world and this certainly is

About black brush algae:
Ghazanfar, Siamese algae eaters eat that stuff.  Their scientific name
is Crossocheilus siamensis (not Epalzeorhynchus any more, Cynthia!).

Liisa Sarakontu                     INTERNET:    lsarakon at hila_hut.fi
Helsinki University of Technology   WWW homepage http://www.hut.fi/~lsarakon/