You might also try terra cotta or clay pipes set in the bottom of the
pool; they're smaller than cinder blocks, and won't change the ph of the
water (which I've heard cinder blocks can do). The key is to provide some
nooks that paws and bills can't get into.

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Coll-Peter Thrush, M.A.
Mangels Fellow in Public History
Graduate Program in History
University of Washington, Seattle
ishriver at u.washington.edu
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