You can put a tang in a goldfish bowl, but it's not going to be happy. Tangs are grazers, and they need lots of space and lots of live rock to find the algae they need to stay healthy. If you crowd a tang into a tank smaller than 100 gallons, it's going to be miserable, and most likely make everyone around it miserable as well. Personally, I wouldn't put a tang in anything less than 125 gallons. If you want something similar to a tang, I recommend one of the dwarf angelfish. Those can be housed in 55 gallons or larger. The people that are trying to talk you out of putting a tang in that tank are people who have knowledge of what works and doesn't work. Listen to them. The guy at the store is trying to sell you fish, and when those fish die, he will try to sell you more. I know 75 gallons seems like a lot of tank, but in reality, it isn't. You will fill that tank to capacity in no time. The larger the fish you put in a tank, the fewer fish it will support. I recommend you stock a variety of smaller, colorful fish instead of two or three large fish. My 125 gallon tank has a 30 gallon refugium, which gives me a total water volume of roughly 140 gallons. Not 155 gallons, because rocks and sand displace about 10-15 gallons of water. In 140 gallons of water, I am able to sustain 14 fish... one fish per 10 gallons. In reality, it's actually figures around one inch of fish per 5 gallon. Cleanup inverts like snails, crabs, and sea stars also add into the equation. Saltwater fish can be territorial, so they need their space. The more space each fish has, the happier they will be.