Clean is slowly, but steadily. 15% and a little muck removal each time. Eventually you might be able to get the fish to take pellets, but it will take some training. Feeding smelt and shrimp 3-4X a week would be better IMO.
If it wont take the prepared foods drop a feeder in the same spot for a few days in a row. Then start throwing some smelt in the mix. he should hit it fast enough that he doesn't know it's not a feeder.
The tank and filtration is fine, you could get away with less flow even. Make sure that the biomedia isnt clogged with crap too.