My concern about sand for cats is that sand is easy to swallow by these bottome feeders, and it might grind their stomach and intestine. But I could be wrong. If they swallow gravel they can split it back out.
i lost a lot of tiges when they were kept in a tank with large gravel
im not saying that killed them but a small bit of grave is alot easyer to pass than a large pebble
when i see my tig eatting he snaps at food and a few bits of gravel come out of his mouch it only take a few large bits of gravel to get swallowed and it chould cause problems
cats swallow whatever is in the tank. Large stones should be bigger than what they can swallow, and smaller ones are ok so long as they can pass, and if they can't pass they will puke it back up sooner or later. Sand is fine. It passes straight through with no problems.
personally i hand feed all my larger cats. they are in with larger (bout 1") rocks but they never really try to eat anything that's not from my hand unless it's floating at the top like my old thermometer.lol.