Thank you for your advice about purchasing pond products. (Point taken,
see bottom.)

I understand that there are two types of bentonite-- Sodium & Calcium.
Apparently, the sodium type clumps more, where as the calcium type, is a
bit more soluble (perhaps better, for some types of filtration.)
See: http://www.sfbakc.org/koienews/clayvsclay.html

I found this (see below) on the site: (along with info on how it makes
the water crystal clear and destroys algae.)

"Wal-Mart has a brand of kitty litter, apparently only carried by
Wal-Mart since it is their special brand, which is clearly labeled as
Bentonite clay, the preferred kind of clay to add to fish ponds. It's
brand name is 'Special Kitty', in a 25 pound red colored bag
(unscented), costing $5 for the 25 pound bag."