I. Tank Size
The first decision to be made when starting an aquarium is, of course, which tank to buy.A standard 20 or 30 gallon aquarium is a good choice for beginners. This gives you enough room to have a variety of fish while still being able to maintain a biologically balanced environment.
II. Picking Fish
Fish size and quantity are the second steps in planing a tank. The size of the tank determines how big your fish can be. When buying fish ask about their adult size so that you know not to buy fish that will outgrow your tank. The most commonly used formula for filling a fish tank is 1" of fish per gallon of water with a minimum of 1 gallon per fish. Overstocking a tank can lead to excess waste which will eventually lead to an unbalanced waste management system.
When mixing different types of fish in an aquarium you have to consider the disposition in question. Fish can be described as "aggressive","semi-aggressive","aggressive while breeding" and "non-agressive". Aggressive fish will be aggressive it is impossible to condition a fish to act non-agressive when its genes tell it to act otherwise. The only reason an aggressive fish would not act aggressive towards a passive fish would be if the passive fish was much larger or able to defend itself. That being said, a general rule about fish size and disposition is that small aggressive fish can be kept with large non-agressive fish but larger aggressive fish will eat anything small enough to kill or swallow. You should always research the fish your buying before you buy it to make sure it will be able to survive in the conditions your able to provide.
iii. Water Conditions
When researching your fish you should find information on their living conditions as well as their disposition. Chlorine, which is commonly found in tap water, is harmful to fish. A dechlorinating agent should be purchased from the pet store and the water should be dechlorinated before any fish are put in. Water pH is another issue. Fish can require a pH anywhere from 5.5 all the way to 8.5 so this requirement should be researched and met before fish are added. Fish that require a pH of 5.5 can not live with fish that require a pH of 8.8 so fish in a community aquarium should have a comfortable and attainable pH margin. Another element of the fishes environment other than pH is the temperature of the water. Only fish who can survive in similar temperatures may live together Ways to alter pH and other information about water quality can be found here.
iv. Habitat: Plants and Rocks
Plants and rocks should be used to provide shelter for your fish. While researching your fish your should find information about their habitat or what common aquarium setup is suggested. Rocks work for all fish and some fish will eat plants so the only other option are rocks. If you find your rocks outside you should stay away from quarries and area's where metal is commonly found in the soil. Rocks with heavy metal or high acidic levels will destroy your water quality and harm your fish. If you think you have a good rock you should soak the rock in 1 part bleach 9 part water mixture over night and then thoroughly clean it until the smell of bleach is gone. Then soak the rock in freshwater over night. If the rock maintains its structure and color and all of the bleach is removed the rock should be safe for use. Pet stores will sell rocks that have already been determined to be safe but these rocks should be cleaned with hot water and allowed to cool before they are put into the fish tank.