I don't think that's a good long term diet. Since you are the boss, not him, you might want to decide when to get him on a varied diet.
The problem with using just tilapia, or just cod, or just shrimp, or just anything that's not designed to provide a varied diet is that:
1) most fish, even piscivores, consume insects, veggies, and crustaceans that are in the prey's stomach. So their diet isn't just fish.
2) fish filet is not the whole fish. With the whole fish, the piscivore is also getting bones, organs, scales etc that might include even more nutrients and minerals not found in the filet
3) frozen fish flesh has the potential to lose a number of specific nutrients that are still in fresh fish flesh, so that's another concern
While 3" might not be the right size to starve him, at some point you may want to train him to eat pellets, and several other kinds of foods. I'm sure there are people who have grown fish on just tilapia, but I am skeptical that it's the most nutritious diet they could have had, and I'm skeptical that all of those fish lived as long with as much color/behavior as they otherwise might have had with a varied diet.