This paint will be fine since you plan to paint the OUTSIDE of the tank. Had you planned to paint the inside, you would have needed a paint designed for constant water immersion.
Most people I know use vinyl backgrounds or foil taped to the back pane. I've even seen a tank with a black garbage bag background that didn't look bad at all. A painted background is a bit more difficult to change, so just be sure you can live with the color.
A word of advice, even though I've never painted a tank.
Tape newspaper to the top and sides, so you don't have any overspray hitting these areas.