in a couple of months it can go from 3.5" to 6-7" you do know that right? if it's in the right environment that will be an easy length to hit. and when once day you see it flick its' tail you can see how fast it covers a 6' tank. if you have the cash just drop it on an 8'(+?) ft tank right now - why end up paying a whole lot more for a tank you will need anyway?
Couple of years ... not months.