Clay for pottery purpose is just as good as any other, especially if
you can get a bulk bag at a reasonable price. Chop and mash it say 30%
clay with 70% any ordinary ordinary heavy loam or sandy soil and you
will have a rather good mineral rich soil for aquatics to get going
in.

You might find plain kitty litter more cost effective. If you search
'smectite' aka 'bentonite' type clays you may find that is
surprisingly cheap and and abundant in many areas... When I go digging
ponds in the garden I quite often hit beds of it in places so I get to
set aside large lumps for potting plants, some water lilies definitely
prefer it...

Clay has some 200 plus minerals and interesting algae suppressing
properties when used to 'cloud' a pond... If anyone is interested to
'try' clay I dig up here, I'll be game to send out dried three pound
lumps (at postage cost) of smectite clay for comparison with other
potting mediums

Regards, andy )
http://www.members.aol.com/abdavisnc/swglist.html

I've never seen clay for sale anywhere. What about clay that is used
for making crafts? Is that safe for aquatic purposes?