Wow, discus are so expensive. You could make a buck. It is no doubt the tetras and cats chowing down. Tank doesn't have to be bare. I raised fry to adulthood and had grandfry, all in the same fully planted tank. However, I only had discus in it, nothing else. Just seperate the tetras from the discus, keep the water clean, and they won't breed fast enough to keep up with demand.