They can get up to 15 inches if memory serves me, so the tank minimum would be 180, even when small as they are REALLY active Tangs. Very aggressive Tangs also! Them and the Sohals have the most aggression issues, even with non-Tangs. Small Clown Tangs also don't have a great track record in captivity, many waste away and die due to being caught too young and not eating enough while at the shippers/wholesalers/stores etc. I would go with a Purple or Yellow or Scopas or one of those types of Tangs for a 90. Anyway hope this helps and good luck!