Filtering Organisms

I have an interesting thing occuring in my 90G plant tank. On the glass
in several places and on a couple of plants there are sponges growing.  I
first saw sponges in my tanks several years ago after I ordered some
plants from Wm. Tricker's nurserey in Ohio.  They now occur quite
randomly in most of the ponds that I maintain and in my aquariums.  When
on the glass they resemble sand dollars, with visible channels through
which water is pumped through the colony.  When on a plant, the colony
blankets the leaf and will eventually grow to include adjacent leaves or
whatever else they happen to touch.  This is more a fascinating thing to
watch than it is an annoyance.  In fact, I currently have the largest
colonies that I have ever seen in a client's pond.  They are about 12" in
diameter and at least 1" thick.  When placed in green water, over time
the water clears and the sponge temporarily turns green.  Just a note
about an interesting bit of microfauna.  It is a neat trick to put a bit
of dye near them and then to watch it course through the colony and then
be expelled again through the collective action of the individuals in the
They never hurt anything and eventually encyst, break up and recolonize
elsewhere according to their own little schedule.  Usually they choose a
dark corner to colonize, but in this case they are right out front in a
well lit tank, in front of God and everybody, for all to see.