I had mixed results with my 18" F8 on a 10gal tank. Of course, it was the one that came with the canopy
. Now I have a dual 24" F12 last used in 1990 on there and it's working much better. More expensive to run - 20w x 2 vs. 15w x 1 F8.
Another piece: 10 gallon tanks are 13" high.