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Re: Aquatic Plants Digest V4 #297

Michael Laflamme wrote:

The rock salt cured your fish.  This is also a known "cure".  By increasing
the salinity of the water you are also increasing the osmotic pressure in
the water.  This change in pressure causes the ich parasite to burst and die
once they end up in the water.  This is why you can give salt water fish a
freshwater dip to kill parasites (and vice versa for freshwater fish)

I doubt that increasing the osmotic pressure of the water would cause the
ich parasite to burst.  It would be most likely to burst if it were in pure
water because, then, the difference in concentration would be the greatest
between the inside of the cell of the ich parasite and the outside.  The
water concentration would be the greatest outside, compared to inside, and,
since water can diffuse rapidly across a cell membrane, the pressure in the
cell would go up.  Adding salt to the water lowers the external water
concentration and reduces the difference in water concentrations between
inside and outside the cell.  Actually, the ich parasite, a ciliate, does
not burst in pure water because it has a contractile vacuole, which pumps
out water as fast as it diffuses in.  If adding salt kills Ich, it must be
by way of a different mechanism than an osmotic one.

Paul Krombholz, in central Mississippi, where we had some rain, but need