I have had my horseshoe crab for about six months now and he isn't growing to fast. I got him at about 2 inches head to tail and he is now maybe 2.25. He cruises around in the sand most of the time but about every three days or so he comes out and cruises for about an hour or so and then he goes back into the dsb. I have seen him eat everything from the dsb critters to krill, formula one, two, prime reef, brine shrimp and detrius. He seems happy if you can tell by looking.
The live sand dollars we have up here are dark maroon and purple, very hairy, and come in sizes from fist size to pea sized. I know that the ones up here eat crustaeceans and the like so I don't keep one because I have quite a few of the former.