I'm assuming you have a 20 inch 10 gallon and that is a 18 inch bulb
You want This glass top/canopy
and this light
both 20 inches. If the light is Aqueon (All Glass brand) then it will come with a bulb. Keep for a spare
If you have an Incondescent hood with the 2 bulbs, it's not a total waste. These are the bulbs people were recommending you buy (if you don't want to spend $40-50 right now)
This bulb is 23 watts
This bulb is 13 watts
Please note on both of the above bulbs it is 6500kalvin .. it's a broad spectrum light and with a medium base will fit virtually all incandescent hoods. It's the ones that I use. Sometimes I use as little as 2700 kalvin because it's a soft light and it dayglow blue hurts my eyes.
Hope this helps. I found out all of this the hard way. And the expensive way..
You could always return the long flourescent bulb and go get these smaller ones and use the same hood. A versa top is recommended with all hoods so the bulbs don't get wet and burn out.
as a second thought, this is a FULL HOOD.. no need for a versa top. it has glass below the light so it's got a versa top built in. I like the seperate items more, you can mix and match and replace just 1 part if it breaks