I use barrel filters and the water can drop 6" without a problem.
Since the bottom drain is plumbed into the side of the first barrel
that is as low as the pond water level can be pumped out by the pump.
If you wish to use a filter like you describe you might want to run a
water line and hook up a constant level switch for your pond. If you
have chloramine in the water that brings up another problem.

Regards,

Hal

On Sat, 8 Feb 2003 11:30:30 +0100, "Pierre de Ponthiere"
wrote:

I agree completely with your explanations when the connection is "bottom to
bottom".
But a lot of filters like Clean Pond 12 described at
http://www.verhaertnv.be/pdf/8A.pdf have the rim of the first vessel 1 or 2
inches below the maximum level. (The second vessel is filled by overflow of
the first one thru a vertical slit visible beteween the the 2 vessels). It
results that the pond level has to be maintained constant which can be a
problem during vacations...