I'm contemplating the issue, but need it to coordinate nicely with the rest of a new architectural interior.
Mildew resistant paint however contains a fungicide/mildewcide chemical. Do you really want that anywhere near your fish tank?
And I'm not sure that a sheet of plexiglass stuck on the wall would necessarily be the most attractive option either, unless finished and detailed well.
It might look reasonable if I tiled it, and used large polished tiles with minimal groutlines, or I could install a large coloured glass backpanel as well.
Another option would be Polished Plaster, which is also known as Venetian Plaster. Some of my guys have recently used that while doing some high-end bathrooms, and used it within the shower room/wetrooms as well even.
You can buy plastic decorative panels as well which look very good, such as those made by Trespa.
http://www.trespa.com/
Otherwise, maybe I should just look at buying some extra kitchen doorfronts to match what I'll use on my stand, and attach those to the back wall to create a surround around the tank, with the joints sealed with silicone.