I have had my naso for a little over a year now and he eats anything i put in the tank. I may get lamed on this but he loves flake and brine. You could try broccoli (believe it or not mine loves it) and or romaine lettuce.
You could also not feed it for a day or two and he will end up going for what ever you stick in the tank. Formula 1 or 2, turns his head at it.
Keep trying, it will be either starve or take what you get kind of thing.