I've got a baby tusk myself. Mine is slightly bigger, but still a juvy. Chopped up mysis works. If you want, take some frozen mysis and run it over a grater to make it smaller. Mine has also taken to pellets right away (not cb fwiw). I've been feeding tdo and ocean nutrition pellets, both small. The tusk gobbles both up quite readily. I'm going to have to grow mine out for some time before I release him into the display. I've got a cb marine Betta in the display from biota that arrived about the same size as yours. He's probably doubled in size over the last 6 months, though you wouldn't know it as he's always hidden. I really shouldn't have released him in there but at the time he was the only fish. I really should have grown him out elsewhere as well.