It doesn't cost that much to set up a nano tank. You can do it for under $250. To me, $250 is not expensive, and that's including corals and invertebrates. Also 6 gallons is not too small. I've seen salt water nano tanks that are under 2 gallons. The only thing about these small nano tanks is that you should really have one or no fish at all. If you do have one fish, it needs to be one of the smallest you can find. Yes, they do require you keeping an eye on them, but what type of fish tank doesn't require that? If you can keep a fresh water planted tank, you can keep a salt water nano tank.