Probably the bulb you have is about as good as any that you might use if brightness
and fairly good color are wanted.
I have used T8 on one tank now for 4 years and I find no one bulb that gives all the
spectrum plants need however. They can have lots of one, but not both red and blue.
The Colormax has a fair amount of both but to me gives the tank an awful hue.
The Zoo Med "Flora sun" has both, but lacks brightness.
Other than those I have yet to find a bulb/w red spectrum.
I have two 10g tanks and one has the T8 on it, but I do two bulbs on there. Two different types.
They are on separate timers and one is on for 9 hrs and the other in the middle of that is on for 3.5 hrs.
Since you didn't say 20 long I'll assume it's the regular tall one so your 4" taller and
that to me says one T8 bulb will only grow Anubias and Java Fern in there.
There is a list of plants which will grow in low light on the low tech section of the forum.
There may be other plants on there that will also grow in there. Some med light
plants will grow in low light, just slower. But I'd check those on the list(that you are interested in) first to see if they are listed as med light as those likely won't grow in there/w that one bulb.
That is the only single bulb T5HO fixture that I know of at this time. Unless you look at horticulture fixtures and the Sunblaster/w reflector is one bulb of T5HO.
Amazon.com : Sunblaster 904296 NanoTech T5 High Output Fixture Reflector Combo, 2-Feet : Plant Growing Lamps : Patio, Lawn & Garden
It will give you med light in that tank. Don't know if this would appeal to you but you
would need to raise it up some for being too much light. But cost effective it is.