A convict might work. I would make sure the convict is large enough to defend itself, have plenty of cover, and rearrange the hiding spots right before you release the convict. Then turn out the light on the aquarium and the lights in the room for a while.
Under some circumstances, some jewel cichlids can turn into killers. I had one in with a pair of breeding firemouths, each of which was slightly larger than the jewel. The 2 firemouths battled the jewel constantly. One late afternoon, I came home from playing football with my friends, this was in the late 70s, I was just a kid. Both firemouths had their fins and tails chewed off completely, eyes eaten out. One was still alive, trying to get away, the jewel was holding on to his tail like a pitbull. I was pretty horrified.
They aren't usually that nasty, but that was an example of what they are capable of, it's stuck in my head for 30+ years. That was in a little 10 gallon aquarium, my first one. My parents called it "the fishtank of horrors". I made a lot of dreadful mistakes cramming cichlids into that little aquarium. At least I kept the water clean. I think the 30 will be much better suited for one, and convicts are usually tougher than firemouths.