Aquarium decorations serve a practical purpose as well as adding color to a fish's home. They provide shelter and relieve stress and boredom for your gilled friends. There is a wide variety to choose from when finding decorations, and a few important things to remember.
Pick a theme. Some of the more popular themes include: Greek (columns), Asian (shrines and pagodas), "ocean-bottom" (sunken ships and treasure chests), and comical ("No Fishing" signs, etc.). There are very few limits for aquarium decoration options, as long as you pick a safe product. Resin and ceramic ornaments are safe for aquariums; make sure there are no sharp edges and avoid any wire or metal parts (because of rust). All pet stores carry some variety of decorations; many of the bigger stores, like PetCo, will carry a larger selection on their websites that you can order. You can find safe ornaments in discount stores and craft shops as well. Craft stores also carry a variety of clean river rocks and glass stones, if you want to liven up the aquarium gravel in your tank. There are several pet accessory stores online, such as aquariumguys and fish, which carry a large assortment of general and specialty decorations.
Silk or plastic plants make an excellent natural background and provide plenty of shelter for an aquarium; regardless of the theme you choose, it's usually a good idea to include a plant in your aquarium decorations. Fake plants can be easily found in craft and discount stores; however, a pet shop or online pet accessories provider will carry fake plants that are already weighted and free of exposed wire. If you want something more organic you can find driftwood and coral kits on specialized sites like aquariumguys and thatpetplace.
Keep perspective in mind as well. The fish have to have enough room to swim freely. If you have a bowl or small tank (1.5 - 3 gallons) you probably won't need more than a plant or single small decoration, such as a pillar or treasure chest. Be sure if an ornament has openings, that your fish will be able to swim through them without getting stuck.
Be sure to pick up water conditioner along with your aquarium decorations, if you don't already have it; all pet stores that carry fish carry conditioner. This will remove chlorine and other harsh substances from tap water, and, along with the right embellishments, ensures your fish's health.