At 30''s or so it is still a juvenile and requires more frequent feedings than an adult. The tuna is a good diet but not complete. Add fresh squid and octopus to the diet, but remember that with the higher oil content you'll have to do a more complete cleaning of your skimmer more often than currently required.
Do not feed small portions in comparison to its head; as an example, what a half of a Big Mac would look like to your head. For a 30'' eel it can consume the equivelant of 3 jumbo shrimp per feeding. Feed every other day. Once the eel exceeds 42-44''s it should have considerable girth and only be fed a full 12'' fish once every five days.
Blue Trig,
Your Zebra is primarily a crab eater and while shrimp can be added to the diet, they will not provide the amount of calcium it requires for good health. The shell contains large amounts of calcium. If it eats fish, it means it is starving and is in trouble. Check with your local LFS if they stock LR. Gorilla Crabs are often pests that arrive with LR and most shops are more than willing to get rid of them.