by painting it. either by spraypaint or normal black paint. some people also buy window tint from walmart or just tape black background to the back. mine i personally painted with normal paint.
I use Oceanvisions adhesive backgrounds. You apply w/soap & water like car window tinting. Sticks really well & you can apply while tank is up & running. I also run a thin bead of sealant around edges. Believe they come in black & 2 shades of blue.