From my freshwater experience I found that snails need absolutely no work on your part to stay alive—in an established tank they have ample algae and whatnot to feast on. With regards to crabs, I found that they mostly enjoyed a diet of the pellets I fed my bottom feeders as well as junk in the gravel. Without the bottom feeders, I found the crabs did fine on their own. I would be careful about actually "feeding" the crabs regularly, since they probably will not be able to consume all of what you feed them. You could supplement their diet from time to time as mil suggested helping keep them healthy—just make sure it doesn't rot at the bottom.