30W single Bulb?
This tank is how long?
I'm assuming the 25 gal to be a 25T and 2ft long.
2x20W would be ideal.
I think this tank is actually a 25 L, perhaps the same dimensions as a 29
gal, but a little shorter.
If so, you can use another bulb, maybe a 20w if it'll fit.
If you have a customer wood hood/box, you can raise the lights about 2-4
inches above the surface to lower the intensity and also to spread out
the
light better so it'll have the effect of slight less light, so you can do
that with 2x30w if you wish.