Some of the softies do in fact eat nitrates.
The problem is with the balance and really figuring out how to measure it. Yes, the ocean has nitrates usually ranging from extremely low to none-existent. But even a measurement of 1 in the ocean is a constant measurement of one. Meaning that that wild softy always has at least some nitrate to feed on. In the fish tank though we can easily strip the tank of usable nitrates until that undetectable range actually does become 0. At that point that domestic softy doesn't have any nitrate to feed on.
The other issue is we can't really measure happiness in corals. We can measure growth and extension, which is not always the same thing as happiness. In fact, some marine life responds to stress by going into rapid reproduction. So some creatures that are seen as "thriving" because they are reproducing constantly could in fact be being tortured. Other ones that are seem as "upset" because they aren't growing quickly could be perfectly happy and content. We tend to force our standard of happiness onto other creatures.
The key is really balance. Don't let levels get to a toxic level, but don't strip the system bare either.