>>986745Well, you should ideally prepare the water with an OR device (or whatever it is called in English), you obviously have to add the salt mix, which can also add up when we're talking such amounts, you might want to add various trace elements...on top of that, such a large tank requires tons of lighting to keep your corals healthy, which can lead to pretty high electricity bills.
However, a 200g is about the minimum size for many lively, fast-swimming fish, and would in fact still be way too small for fish like Acanthurus sohal (pic related), various Pomacanthus species etc.
It's no problem to keep a smaller tank stable (a 55g should work perfectly fine in this regard), but it's by no means unusual to invest in tanks of that size when it comes to saltwater, as it opens up way more possibilities when it comes to your selection of fish.