Would they get too big for a 120g?I had one successfully and it was very fun until it jumped the tank (through the smallest hole). One of my LFS's has natural tanks with sand and corals and that's how I got a healthy one. As for an alternative how about an engineer goby. They get massive and are as big of sand movers and buriers as a diamond but are also far easier.
Here's one uncovering all my buried pipes.