LOL his sailfin is small, it's not going to die anytime soon, but it could eventually. You'll be fine keeping it for at least another year, but IT WILL EVENTUALLY OUTGROW the tank and get severely stressed, so don't get attached (or start saving up for a bigger tank). Agreed they should not both be in a 55 though (yellow should be fine). Sailfin is fine in a 4x2x2 120 gallon - the width makes up for a lot length. Like everyone else said, I'd just get it over with now and take the sailfin back for credit and get something else (not a tang). You'll have much happier tank inhabitants.