I worked on ORAs aquacultured mandarin project in 2007. From my experience, there's no need for a large tank. Just feed proper diets and they're fine. With many commercially available pod products, they've never been easier. I've also never seen a farmed dragonette refuse small pellet. j/sAgree 100% except I’d say get a 125 or give up the tangs. Tangs need that 6’ of length to swim Most 120s are only 4’ a 120 high is 5’ which is marginal for tangs. If you want a mandarin you need the big tank as well. You can’t supply a mandarin with enough food constantly in a smaller tank. Sure they live a while but the will slowly die of starvation in a small tank. Your 40 would make an amazing quarantine tank or sump.