If you can find a SMALL fighting conch, it will also do a great job. A 29g is just on the minimal edge of sustainability for one of these, so you will have to a) find a small one, b) supplement with nori or algae pellets if your tank doesn't have algae on the sand bed or close to it (they do not leave the sand bed) and c) be prepared to rehome it when it gets too large.
Having said all that, they are really fun to watch. Their locomotion is strangely awkward, but they can hop fast when they need to. They clump around the sand bed and bury themselves, so they really keep it churned up.