Put two medium dubia roaches in there last night and when I got back tonight, one was missing. Though it's possible it was brushed up under the rocks. So I put a shrimp and a roach to leave tonight in a place that will show more certainly that it was eaten.

Also, I got the filter from amazon yesterday, so I installed that. No suction cups because I've read a story that it's possible they could eat them, if there's no substrate available. As sometimes they eat stones to help with buoyancy and grinding of food.