If it is a 20 gal Long, you should be alright, It is very rare that they get 8", Ive had mine for a 17 months, and has grown less than an inch, and is about 5-6" right now... she eats well too. She is in a 26 gal Bowfront, but probably has less floor space than the 20 gal Long, and that is what is most important...