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RE: diatomaceous earth

> I was just considering use of a diatomaceous filter when you 
> posted to list. 
> I've wondered whether allowing some to escape into the tank 
> during use could 
> be detrimental to the fish. DE dropped outdoors, for 
> instance, is supposed to 
> be helpful for insect control, including ticks (supposed to 
> kill them, I 
> believe).  Wouldn't the small particles injested through 
> gills or otherwise 
> be somehow harmful?

Fish are almost constantly exposed to living diatoms, as they are an
important and numerous component of plankton, but do not suffer from
silicosis.  There are many reasons:  1) fish have specialized structures on
their gill arches called gill rakers.  These bony structures serve to
"strain" the water before it passes over the gills, and plankton-feeders
have very numerous, close-set and elongated gill rakers that strain out and
collect plankton.  They swallow the collected plankton.  2)  Unlike
lung-bearing animals, fish have the ability to "back-wash" their gills,
where they reverse the flow of water and wash off any particulates that may
be stuck on the gills.  Ever see your fish "cough"?  3)  Gills are covered
with a thick coat of mucous, which is always being produced.  Any diatom
that gets imbedded in the mucous coat will eventually become sloughed off.
Ditto for the mucous-lined alimentary canal.