thanks his belly is starting to turn yellow now.how big do they grow.my tank is a 55g tank.im upgrading next year to a 6ft tank to give him some room.he is defo the boss in the tank.he is not a bully thoe.
At six inches the fix has really already outgrown your tank.
150-200 gallons is a good size for this fish.
It will get a foot long easy and it's deep keeled and fast swimming.
For your size tank the Kole (yellow eye) tang will fit for it's entire lifespan.
They get more aggressive the older they get.
Personally I say judge it by behavior and health. If he is healthy, has good color and is behaving normally, not overly aggressive, has plenty of room to go back and forth, then he's probably okay for a little bit longer.
Tangs in general need a lot of swimming room. Some not as much as people think do to grazing behaviors. The hippo is one that really does like its room though. I would say this one is going to be unhappy very soon.