I've kept cardinal tetras and they make for a spectacular display. In my experience though, the discus will hunt them down and eat them all in 2 - 3 months. They don't seem to eat rummynose at all, which are a better schooling species. The downside to rummynose is that they fade in a bare tank. If yours is planted, they would be something to consider. Really like to see how the hatchetfish do with discus.