The serious answer for the Tangs is that they really need a lot of space to swim or they become stressed. Stress weakens a fish's immune system. Weakened immune systems open fish up to diseases and other health issues. Tangs are very susceptible to Ich, and Ich is most often started in stressed fish. If you had three Tangs in something smaller than a 125g, you are looking at an excellent chance of Ich and I'd also bet on having territory/agression issues.