Originally Posted by AquaRick
It's my understanding that tangs have rather large and long digestive tracts and is needed to break down plant matter (nori/seaweed). Carnivorous fish on the other hand don't have the longer digestive tract (compared to body length) as they do not require it to absorb the nutrients from their meaty foods.
Very similar to other herbivores such as cows etc.
That is accurate, but can be relative. For example, cows and horses eat basically the same vegetable matter, but cows process it more fully. If you have raised both, the difference is obvious when cleaning up after them. Surgeonfish have a very long gut, but only one stomach. As a result, they produce a lot of poop for the energy they get from their veggies.