It's not really much to look at.... No offense but it looks more like a picture frame...
I will admit though, it looks like you have a decent technique down.... As far as finishing it.
Are you looking to have a 3D frame around a 2D background placed on the outside of the tank? Or were you trying to see how a 1st attempt 3D would look?
You really have to decide on what "look" you are trying to achieve... Do you want your tank to look like a slice of nature placed in your house or a bumpy piece of painted styro place on the backwall of your tank?
Remember that 3D objcts or a 3D view will show great contrast up close and the further back one looks a the 3D view, the objects, whether large or small, will appear to fade in contrast and color.... So if you want a nice 3D affect, then you will need more contrast.... (i.e. difference between blacks and whites)
Keep us updated on your Journey though...