ok so i had a 24" 10.0 reptisun t-12 uvb 20 watt.. i ordered from petmountain a the same thing, except it was 18 watt, and it said a t-12 but is a t-8?? i put it in my fluker fixture, and it works? does it make a difference, since one is fatter (t-12) than the other??
Ok my two cents cause I just went through the same thing. T 12's are not the same as T8's. The diameters are different, the wattage is different, and sometimes the post fluorescent fixtures will not hold or work the voltage.
For instance when using an old T12 fixture your T8 may not work because T8's run at a lower voltage. If you do get contact and lighting, you may find that your T8 light may not last as long in the T12 fixture.
If you have a T8 fixture you cannot run a T12 at all, due to voltage.
They now make dual fixtures that run both T12's and T8's, with voltage currents rated for both lights.
However, if you are using your T8 in a T12, then it should be ok, it just may have issue with bulbs burning out sooner or running a little dimmer.
To confuse you all the more there is also T10 rated lights which will work in all T12 fixtures.