Just about anything has been used successfully. Sand, gravel, cat litter,
cow dung, clay, and mixtures of the above. I use some of my sandy
gravelly soil with some plant food spikes buried in there where the
phosphorous can't leech out and cause green water problems.

all of the above are ok.....potting soil is NOT OK It is too light and will
float qway....Make sure if using cat litter it is NOT the clumping kind and
that it is well rinsed.....I personally pot in 1-2 inch gravel, and let the
fish fertilize them Jerri

http://www.fringeweb.com/Ponds/JerrisPond