These guys are starfish eating machines, but are a poor choice of a solution towards mini starfish infestations as once the problem is solved the harlequin will then starve to death.
When it comes to asterina star outbreaks, they can easily be siphoned out. When it comes to bristle stars, large numbers is a typical 'life stage' of some aquariums and an indicator of how much 'food' is in the water column. More food for anything to eat will result in more of the animal!
I had a huge outbreak of bristles in my tank in the early months-year. Hundreds and thousands of arms would come out everytime I fed the tank. As I began ratcheting down on the nutrient issues in my tank, the numbers dwindled until the only starfish in my tank is my green serpent.
As for using animals to battle issues in the aquarium, I will use my lawnmower blenny in my reef tank to help solve the algae issues that were going on. Lawnmower ate all day long, but still algae because the nutrients weren't addressed. It is still in the tank today, but sadly seems to be withering away...something that happens to lawnmowers even with algae/seaweed available in the tank.
