Your baby girl is so little! A 20 gallon should be good for quite a while. I bought a 10 gallon tank when I first got mine about 4 months ago. She was approximately the same size as Betty there. I bought it thinking "I'll get her a new home after I move in August, that seems like good timing." But man...they grow waaaay faster than I was expecting. I may have to upgrade sooner than I originally thought! 20 gallons should work for you for at least a year or so, I would think.