I would say maybe to that, If you had like a 55 or 75 (still small tanks for a Hippo). But with a 30g, you really can't have any tangs, baby or not. Tangs are fish that need a lot of space to swim around.
Oscars and pacu aren't really something you can compare this to either. Oscars are indestructable, and freshwater in general is a completely different world of knowledge and experience. Honestly, the only thing I can think of that SW and FW have in common is the nitrogen cycle. Other than that, comparing the ocean to a lake/river etc ecosystem is like comparing an anarctic tundra to the rainforest.