The last time I painted the back of a tank I used spray cans of acrylic paint. Much neater looking than with a brush, but for a 120 gallon tank, it took me 3 cans to get a dense, uniform coat. That's why cheaper is better, in my opinion. A ten gallon tank shouldn't take more than one spray can of any kind of paint, and, if you want it to look good from the back, maybe glossy black would be best. If I were to paint a 10 gallon tank I would also lay it on its front, so the back was horizontal and level, making runs much easier to avoid.