Remember that sand is a natural substrate in ponds, etc, so I think it is very likely your catfish ate them. Many catfishes do this, to begin with. What kind of cats were they? They are also sensitive to water quality.
Green - Interesting about the Cichlids. You aren't the first to say that Africans didn't bother the snails... HMMMM... American Cichlids do it for me, too - and they do bother snails big time! lol