Call the LFS and see what they were feeding it before you bought it.
All I can suggest is variety at this point.
I've never owned an L-18 but I've read on MFK that their primary diet as juveniles in the wild consists of some sort of biofilm that can't be replicated in aquariums (or something along those lines anyway...). I think this is why L-18s have a reputation for being sensitive and sometimes die young in aquariums.
But try other veggies/fruits/etc. Maybe water melon, or mango, or peach I hear good things about (as far as stimulating appetite). Goodluck
edit: and try adding the food after dark/lights out, it should be more active then.