skip the brush unless you want brush strokes.
If you are able to take the tank out doors spray paint in my opinion give a neater look, but you must do it correctly. Meaning no drips. Step back and go with smooth slow even coats.
As mray stated the above procedure is for an empty tank. If your tank is stocked then go with a roller. HD sells these really skinny rollers that are great for this application, and if you go with this method make sure you get the long handle applicator.
I have painted most of my tanks with the roller!! With exception of the 20g., that I did by spraying.
Both are just as simple.
Best wishes on a job well done.