If the tank is having a problem with algae or has a kinda old light, growing them floating will produce the best results. Under good lighting my hornwort grows around a foot a week.
Greens are a great way to cut down on ammonia, but they will still need protein from the flakes/pellets. Ammonia is a direct result from the breakdown of proteins by the body (even our bodies produce ammonia). Ideally their meals should consist of 1 part protein to 4 parts greens/veggies. This will help reduce ammonia waste as as they wouldn't be digesting as many proteins but mostly plant matter. Shelled peas, spinach, chopped zucchini, and kale are great vegetables for them. It's also great for their digestion and will help them not munch on your plants as much.