That tiny baby tang is gonna grow quick. I've had mine for a year and he is at least 4 times bigger than he was. I also have him in a 6 foot 125g tank and I still feel bad for him in that tank. these guys grow big and get fat too. Mine is quite solid and plump and i can't imagine him in a biocube, even when i first got him.
I can only wish you good luck though! And hope that big tank of the future happens sooner than later.
125g Reef Yellow,Tomini & Blue Tangs,Watchman Goby, Squareback Anthias,
60g Rimless Reef- B&W clowns,citron goby, mandarin, scooter,Barnacle blenny,firefish
125g Malawi Cichlid
90GFOWLR Red & sixline wrasses,Flame Hawk,Squirrelfish,Engineer Goby,Coral Hogfish