Aquascaping is the part of setting up an aquarium that allows you to get the most creative. There is no right or wrong way to do it as the bulk of the decision are your own personal preferences. There are various options open to you and this is a guide of key points that you may want to consider.
Design and Structure
Make your tank look natural with distinct sections. Have an open area for food and free swimming. It's a good idea to research the needs of your own fish to assist in the decision making, after all it will be their home. Give fish hiding places and items that can be used as markers to help them designate territories.
Gravel: The color can be whatever you prefer, choose one that compliments the fish. Gravel looks best if it's sloped to climb up toward the back of the tank. This can be easily done with a straight-edged object like a ruler and will give the aquarium a look of depth.
Coral and Rocks: Coral comes in varoius colors and can liven up any aquarium as well as provide markers for territorial fish. Be sure to use ones that were specifically made for fish and are freshwater aquarium safe or there may be leakage of minerals or other substances that could adversely affect the water. These usually look good in the back or middle of the tank.
Caves: These are ideal hiding places. Fish need hiding places to help relieve stress, run from more aggressive fish, or simply be a place to feel secure in. This is especially true for bottom-feeders.
Background Wallpaper: The wallpaper choice has little affect on the tank inhabitants, you may even want to add wallpaper to the tank sides. Given the ease at which it can be taped to the tank you may want to change it periodically to give the tank a new look. There are various background themes available including aquatic plants, ancient ruins, reflective, and deep blue.
In addition to these standard backgrounds you can opt for hanging submerged leaves. The hanging leaves look more realistic than wallpaper and the fish can interact with them.
Plants: Use various plant types in different parts of the aquarium placing the shorter and smaller ones up front and the taller and larger ones towards the back. Plants are important because like caves and large rocks they can provide hiding places and are essential in marking territories. Plastic ones are easy to add because their bases have a cup-like feature that you fill with gravel to anchor them down, but you may want to use live plants in your aquarium.
Aquatic Miniatures: Adding these odd little decorations is another personal choice. Most pet stores have a variety of sunken ships, treasure chests, divers, funny signs, and floating toys. There are even seasonal ones for Halloween and Christmas.
You may browse through this gallery to see how other people have chosen to design their aquariums.