A few observations from experience. At first I used large marble sized
rocks on the top but found that is you use a lot of them they become
like cement because of them settling and the plant roots. This makes
it very, very hard to fertilize and to split the plants to propagate
them when the over grow the pot in a few years. I switched to using a
light covering of pea gravel. If you have pond lillies then this will
be a issue because pond lilies are fertilizer NEEDY - a minimum of
twice a year plus they will outgrow the pot and will need to be divided
a few years down the line. For my newer plants I have used a mix of
cheap unscented cat litter (it's just clay, from walmart) and a small
amount of 'clean' yard dirt. (where there are no pesticides). I
mix this up in the pot and top it with pea gravel after I potted the
plant in it. Add a few fertilizer tabs and it worked so far. For
other shallow plants I have just used pea gravel mixed with a little
cat litter.