Nice tank man! I have kept rays with discus before. My tank was 100 gallong long. It worked out but I had less plants and wood than you. I agree that it can be done but you have to give the ray room to move around as it pleases. Its constantly sifting sand and throwing it this way and that. Hystrix rays dont get that big compared to others like motoro and leopoldi so they should leave the discus alone.