Ultimate Koi Clay appears to dissolve when mixed with water, but it only goes into suspension. The clay is made up of billions of microscopic particles. Though it's difficult to comprehend one tablespoon has approximately the same surface area as two football fields. Below is an image of bentonite clay at different magnifications up to 20,000 times its original size. As these particles spread out and settle in the pond they adsorb (to gather on a surface in a condensed layer) and absorb (take in or soak up) toxins from the water. During this process minerals are released into the water.
The best explanation I've seen on how calcium bentonite clay removes toxins comes from clay expert Ran Knishinsky "Technically, the clay first adsorbs toxins (heavy metals, free radicals, pesticides), attracting them to its extensive surface area where they adhere like flies to sticky paper; then it absorbs the toxins, taking them in the way a sponge mops up a kitchen counter mess."
After that occurs the clay can be filtered out of the pond via your mechanical filter or vacuum. If left in the pond the toxins will continue to be bound.
Clay at different magnifications up to 20,000 times its original size.