[lcstorc]

I know nothing about convict tangs, but IMO 3 tangs in a 90g is iffy at best. It can be done, but i would be careful. Find out what type on tang the Convict and the blue are. Everything I read says two tangs of the same body style are very likely to fight sometimes to the death. To minimize get the new tang of a larger size than the established one, add the new fish when lights are out and re-arrange your rock, This means both fish will need to establish their own territory.