Get the 18" model and it should fit your 10g fine. If you are unsure of single bright vs. double bright I vote you get the double bright. You can always cover a few LEDs with tape if it is too bright. But I find the single bright is enough light in my 20.
I have to leave the cavity for the root structure of the plants. They are the biological filteration for the tank and very a very thick root structure. 










