Installing a false back in your aquarium is a terrific way to conceal unsightly equipment such as filters, heaters and thermometers. You can also use one to highlight your aquarium décor or to show off your colorful fish. There are several types of materials suitable for your aquarium's false back.
Panels
You can easily insert a glass or acrylic panel in your tank to create a false back. Colored acrylics can be special-ordered but tend to be very expensive. Your best bet is to purchase a clear panel and spray paint it using a non-toxic paint made for use on plastics.
If possible, buy your insert pre-cut to your tank's interior dimensions. If you have to cut the piece to fit on your own, use a saw blade with a high tooth count, as it will give you the cleanest cut. Cut the piece slightly larger than what you need and go over the edges with a router to smooth out jagged areas. You'll want as smooth an edge as you can get so the piece will properly adhere and seal to your tank. Use a non-toxic silicone sealant or adhesive to attach your false back.
Styrofoam Pieces
Many aquarists take Styrofoam and mold it into various shapes for the false back. This gives you the ability to create caves and tunnels if you wish. Your fish will love it, and it will add an extra dimension to your aquatic environment.
Once you have it shaped to suit your taste, seal and coat with waterproof, non-toxic materials. You can apply an epoxy and cover it with sand or gravel while still wet to give it a natural look, or seal and use colorful paint to add to a whimsical tank.
Expanding Foam
Many aquarists have used expanding foam as a cheap and easy false aquarium back, such as the spray polyurethane insulation foam. Unfortunately, those foams only come in an ugly yellowish color and could potentially be unsafe for your fish.
A product called "Handi Foam" is an expanding polyurethane foam developed to landscape koi ponds. It is black in color and non-toxic for your fish. It adheres to virtually any surface, and if you aren't happy with the results, you can peel it off without causing damage to the surface it's applied to.
You can apply the material directly to the back panel of your tank, or onto a false panel to be inserted into the tank. Create caves, imbed stones or pieces of coral - use your imagination to dress it up however you like.
Adhesives
A word of caution regarding adhesives when you are attaching a false back to your aquarium. There are many silicone sealants available on the market, and many may be suitable for use in your tank. DAP makes a silicone sealant specifically for use in aquariums, and there is a similar product called Perfecto.
Be wary of sealants designed for use in the kitchen or bathroom. Many have added substances such as fungicides that could be toxic to your fish if they were to dissolve into your aquarium habitat.