I have one that is extremely healthy through three years. While I feed the tank with assorted frozen foods...squid, octopus, clams, mussels...he doesn't eat any of the frozen stuff.
He exclusively eats Nori that I buy from the Asian Grocery...but also loves pellet foods that I toss in every now and then. He doesn't prefer it, but sometimes I hide brown/red algae in the clip with the green. They try to eat around the brown/red, but get some of it unintentially. I figure the variety is good for them.
So...that's how I keep mine healthy. Good luck.