If you have a fixture with an electronic ballast, it will work with either T8 or T12. <The endcaps & length are identical between T8 & T12, so they are physically interchangeable.> The electronic ballast and the T8's are both more efficient. They might cost a few bucks more, but you will save that in electricity in no time. <A T8 is about 15% more efficient than the T12.>
If you get an old style T12 with an electro-magnetic ballast (or worst yet, the big old magnectic ballasts), you will have to swap out the ballast to run T8. < Not only are the old ballasts big, heavy & inefficient, they also hum ... I would avoid those at all cost!>
T8's are the most cost efficient way to light a tank. Plain T5's are prob not that much better & is not worth the extra cost IMO
. If you want to go up from T8, get T5-HO
's with a good reflector.