If your tests are good for a couple of weeks, you could start with some zoanthids. That way you would have a little color, they will "encrust", and are extremely hard to kill.
If the trigger won't eat them, a cleaner shrimp or two would be nice. You have to acclimate them slowly, but then you would have something to pick up any food that gets to the bottom, plus they will be valuable later on for fish cleaning duty.