Hi Phil,
Yes, that's the main problem with keeping large cowries in a tank, they tend to bulldoze everthing else around. Most of the tropical species are omnivores, with the exception of a few genera that normally eat sponges. You aren't likely to encounter any of these in the aquarium trade, and aren't likely to collect them personally, either, unless you live in South Africa or Australia. They generally eat algae, as well as scraping the microorganisms off rocks and probably the glass of your aquarium. They may taste of other things, and some people find that objectionable, but they generally won't feed consistently on anything that you are likely to really want preserved. A different species, Cypraea arabica, was kept for many years in a tank, being fed a diet of lettuce and clams on the half shell... You might try that. I suspect that others who have tried keeping the large cowries without attempting to feed them are probably the same ones that had their starving cowries "tasting" of too many things that they wouldn't normally eat.
Glad to hear he's doing OK!
Cheers,
Don