Most likely they will work and give out more or less the same light as the T12s.

But:
- make sure the T8s will go in your fittings (look at the pins at the end)
- there is a small risk they won't light (probably will be OK if the pins are compatible)
- and you're wasting money/light buying the cheapest tubes ($1 tubes get dimmer after a thousand hours or so, $3 tubes run at full brightness for 20,000 hours)

shrubs_n_bulbs, what # on a package of bulbs determines how fast the bulb will lose its brightness. dale

Dale, this information is often not shown, or not shown prominently. It may be listed as lumen depreciation but usually not since that doesn't "sound good". You will find a note on the better tubes to the effect that they lose less than 10% or less than 5% of their brightness. You will commonly find a term called "mean lumens" in tube specs, this is the output at 40% (I think) of lamp life. You want a number that is as close to the "initial lumens" as possible. It is fairly easy to find mean lumens at 95% of initial lumens in a T8 fluorescent tubes. A very long life, which is always listed, is a good sign. Look for something quoting higher than 20,000 hours and you will almost certainly have very good lumen maintenance.

All this often ignored because even a 30% drop in brightness is hardly noticeable to our adaptable eyes, but it is very significant to a plant. Fluorecent plant lights have traditionally been discarded once they lose, say 20% of their light. In practice they were just swapped out every six months or a year. Using a tube that maintains 95% of full light output right until it fails means you just run the tube until it fails which might be 5 years or more.