What is put into swimming pool filters? Diatomaceous earth.

So how do we get that? Obtain some ocean water. Use a wet mount slide with a cover slip to observe small organisms in the water with a microscope. (Binocular microscopes are especially good for viewing these organisms.)

The small organisms are part of a food chain upon which other organisms are dependent. In the ocean, diatoms (algae and plankton) are the primary food producers and the beginnings of all food chains.

The encyclopedia tells us that a diatom is a "single-celled, microscopic plant that secretes and is enclosed by an often intricate, round-to-elongated silica shell. Golden-grown algae, diatoms are found in fresh and salt water, in moist soil, and on the moist surfaces of other plants; they are the principal constituent of plankton, an important food source for aquatic animals. Most exist singly, but some form colonies. When the aquatic forms die, their shells collect in the ooze on the bottom, eventually forming the material called diatomaceous earth or the more compact, chalky, light- weight rock called diatomite, used in sound and heat insulation, in making explosives, and for filters and abrasives. Most limestone and much petroleum is of diatom origin." That's pretty impressive.

It just blows my mind to realize that immense whales live on krill, which is not much higher in the food chain than diatoms.