Clean the glass first with Windex.
Get ESP (Easy Surface Preparation), apply a coat, wait 90mins for it to dry. This prepares the surface and helps the paint to stick to the glass.
Paint 3 coats using a low sheen flat black paint. Waiting between coats for it to dry.
I used a paint roller, much easier and you can paint it even if the tank is close to the wall. It also allows for an even coat of paint.