First thing that comes to mind is that shellies love to dig, and they won't do that well with gravel. To fully appreciate these tiny bulldozers, give them sand substrate. You can pick up a bag at the hardware store for a couple dollars.
A HOB filter will work great, I also like using sponge filters and powerheads with prefilters. Even my little 10 gallon tanks have 2 separate filters on them in case one of them fails. Keep in mind when chosing your filter that the water should ideally cycle at least 6 times an hour. For a 30 gallon tank that means a total of at least 180 gallons per hour filter rating.
Where I'm originally from (UK) you rarely if ever see HOB filters. I had always used internals or cannister. I now have an Aquaclear on my 20g shellie tank and love it. It's so easy to maintain and the flow is great.
:thumb: To you for having the same question as me. I'm just wondering how the sand isn't also moved with the water or the shellies not annoyed by the sand being picked up when they dig moving it into another pairs territory.
I've always liked a canister filter with modifications to the exits. Currently have a loop exit system on my tank with 5 outlet jets trickling onto the surface. Makes a nice noise, not too much flow but the trickles easily agitate the waste but doesn't disturb the sand either. I'll post a pic when I get round to it.