Mask and spray paint is my preferred method (plasti-dip works well, especially if you might remove it in the future), window/glass door tint is also a great option (Home Depot sells pretty cheap "black out" door tint that I used on my 29 gallon), if you have a "ledge" behind your tank you can simply place a black piece of plastic (or other water proof material, I recently used a black "Life Proof" floor sample behind my 10 gallon. . .