There are two types of bentonite clays - calcium bentonite and sodium
bentonite. Calcium bentonite is generic 'koi clay' and what you are
looking for. Sodium benonite, the kind used in clumping kitty
litters, tends to swell and form icky masses and is generally not used
in ponds for algae and trace minerals.

By the way, the term 'clay' refers to a particular size of soil
particle with specific physical properties, just as silt is a totally
different larger one. And sand is still larger. Particle size is of
primary importance here. They are not interchangable. A mixture of
all three particle types in a good garden soil, usually with some
organic humus thrown in to help structure, is what is referred to as
'loam'.

There have been very good past threads on koi clay on this forum.
Type into a search 'calcium bentonite' or 'koi clay' and read them.
They are very informative and tell where to get generic c.bentonite of
a more desireable form than generic kitty litter.

You can purchase very costly koi clay --if you can even find it
locally in a pet store-- for about $15 or more for 4 pounds, or get
the generic kind u.p.s. delivered to you for about $50 for 50 pounds
(cowboy mining co in texas, amoung others). I got mine out of los
angeles from Laguna Clay - 20 pounds for about $25 to my door, U.p.s.
I had tried generic clay kitty litter and it was unsatisfactory -
never dissolved even after soaking for 2 weeks, then taking a ride in
the blender-, but the brands mentioned that people use were
unavailable to me.

I have read everything I could easily find about koi clay and what it
does. Some believe it helps with their string algae, and others
think it was just coincidental timing. I dont know because my s.
algae was on the way out already when I first put it into my pond.
Others believe koi clay helps cycle new bio filters by providing trace
elements for the bacteria that is missing in liner ponds. And others
sprinkle it on their moist koi food to give the creatures added
minerals for general health, color, and skin luster. Others use it as
a flocking agent in pond water to clear it. Others say its doesnt work
for them. And others put it into water and drink it themselves to
cleanse their systems. (not kitty litter, but very fine bentonite
clays.) As well as a host of commercial applications in mining, water
clean-up, paints, cosmetics, emulsifiers, etc etc.

In addition, someone from Scandanavia on one forum reported that
cattle downwind from Chernobil were fed bentonite clays to purge their
systems of toxins from that disaster, and it worked.

Interesting stuff. Lalu