Decorpart of "A Rich Person's Guide to Aquariums" by Philip Greenspun
The first principle of aquarium decor is that each fish should have a place of refuge. Fish will be happy and healthy if they know that they can escape from predators. Generally the best refuges will include an overhang and offer protection from above as well as the sides. The ideal form of a refuge depends to some extent on the species. For example, most African Ciclids are accustomed to a rocky shoreline. Crevices in rock piles and caves made from flat rocks will be the most natural form of refuge for an African. Discus, on the other hand, may prefer to hide in a tall plant. Catfish and eel-like fish will enjoy tubes.
If you don't care about aesthetics and want something effective, stop into a plumbing supply house or Home Depot. Pick up a selection of PVC pipes and pipe fittings, many of which are finished with smooth edges that won't scratch your pets' skin.
My one great decoration idea that I've not implemented is to enlarge photographs and display them by pressing them up against the back glass of the aquarium. Thus the fish would appear to be swimming amidst the pyramids of Tikal or whatever.
Speaking of backgrounds, keep in mind that these aren't purely for aesthetics. The animals in the aquarium will be happier if they aren't exposed on all four sides. A dark or at least opaque background on the back of the tank increases the fishes' level of comfort.
One thing that I would caution against is the plastic rockscape background from petsmart.com. I put this inside my 55-gallon tank and promptly watched the shy fish disappear to live the rest of their lives between it and the back glass, seldom to be seen again. More disturbingly, a couple of fish died when they were trying to jump over the background and instead made it all the way out of the tank (it is best to have the top of the tank covered as tightly as possible but inevitably there will be openings at the back for filter tubes, autofeeders, etc.). The background looks great but it was a mistake. I wonder if the "Back to Nature" backgrounds from http://www.on-the-rocks.se/england/index.shtml would be better. It seems as though these are glued to the back glass and therefore would not suffer from the same drawbacks as the background that I tried. Sadly these backgrounds, from Sweden's On the Rocks, do not seem to be available in the United States.
I guess "A Rich Person's Guide to Aquariums" isn't the same as "A Sensible Person's Guide to Aquariums."
Instead of putting the background INSIDE the tank, try taping it to the OUTSIDE of the tank, perhaps the back of the tank, which would go against the wall. That just might do the trick to eliminating such catastrophes, like your fish dying behind it.
-- Robert Russell, September 25, 2002
-- Ron Brewster, July 29, 2003