Surprised you found larvae of anything with a 10x turnover . If you've found just one , and used it to feed your harlequins , then I'd do nothing . Wait and see if there's any more . You've got a few days to get any you find out before they turn into bugs .If more show up , then neons or something else small that won't mess with your shrimp might be a good idea . Don't think I'd mess with dunks , I've used them in my pond and they work great , but they're designed for a much larger body of water than a fish tank and might create water quality problems when they start to break down . Maybe if you cut one up and used a small piece , but adding a few fish is a better solution IMHO.