Are you going for 'rough' looking stripes or hard edge/angular stripes, firstly?
The issue you're running in to is two dark colors next to each other. It's going to be harder to see.
May I make a recommendation and simply swap the colors.
Paint the entire tank the dark green, then hit the tank with a slightly brighter shade of green through drybrushing to pick up the edges/details. Then go in and use the black for the banding/striping pattern. Then drybrush the black areas with a medium grey to give them definition.
It may yield better results.
Just a suggestion. Try that out and see if you're happier with the look. Best of luck sir, and I hope it helps.
Take it easy.
-Red__Thirst-