Which kind of paint for the background?

I wouldn't spray a tank that was up and running in a place I had to breathe, so you'll want to use a roller or if you have to a brush. I use acrylic latex that I mix about 30:1 with a clear epoxy that has a working time around 20 or 30 min. Figure on doing 2 coats.
 
You can tape a black sheet of plastic on the back.
 
You can just use black poster board and duct tape. Cheap, efficient, looks great and easily changed.
 
painted it with latex mat black, looks a lot better than before. Already had the supplies figured I'd just paint it.


upload fish 2.jpg

upload fish 1.jpg upload fish 2.jpg
 
painted it with latex mat black, looks a lot better than before. Already had the supplies figured I'd just paint it.

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Latex will work well for a while, but it doesn't adhere well to glass. Moisture can over time work its way between the paint and the glass and cause bubbling. It's really bad in SW or even really hard FW because of mineral creep. That's why I add the epoxy.
 
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