Black rim on top of tank. How important is it?

[mr.wilson]

You need to establish the thickness of the glass first. If it's 3/8" or 1/2" glass, it should have a cross brace at the centre. If it's 5/8" glass, a eurobrace is a plus, but by no means necessary.

As Delphinus mentioned the black plastic cross brace is structural and not just to support a light or glass top.