Regular soil out of your garden, specialized
aquatic soil, or plain, unscented kitty litter will
work (rinse really well). If you have koi top the soil with large pebbles to
stop them from picking it all out.
Potting soil from the garden center usually has things that float in it. Those
little white thingies or bits of woody stuff. You'll be netting it out of your
pond forever... in my experience
;-)

kathy