If your gonna upgrade you must get a 125 im a tang lover, and we have a 210 in the office for triggers,you can get a 75 and get away with a baby tang for a year or 2 but try and buy the equipment used,seriously if you can get a deal its worth it my wife wont let me get anymore tanks even though i'm in hawaii with her now bored lol and she wont let me play golf more than 2 rounds...but this guy lives likes 3 blocks from my cousin in NYC and he's selling a 120 gallon 5 feet long tank with stand practically new with a 30 gallon sump wet dry and fancy canopy and lighting etc...for $499 thats prob way less than your gonna spend for a new 75 or 55..try to get deals in your area or buy equipment online is the best advice I can give you. Six line and Mandarin is fine I have them in my 125 Reef for almost a year everything is cool.