Bloggang.com : =Neo= - How to Purchase a New Aquarium

Steps

Measure the space you decide on, and write the measurements down. People often mis-remember the numbers, and end up buying the wrong size.
Go to a large aquarium supply retailer either in person or online, and browse the different kinds of tanks available. Tanks come in acrylic or glass, and there are pros and cons to both.
Choose a stand/hood (lid) that will fit the tank you like. Make sure that everything comes in the size of your measurements - some types only come in specific size ranges. Buy something one inch smaller (more or less) than your measurements, to allow for cord space, filter access, etc.
Decide whether you want to go with a coldwater (Goldfish), tropical freshwater, or saltwater aquarium. All three are very specific to the kinds of fish they are suited for, and require slightly different setups.
Ask what sort of deals the seller may be offering for new tank setups. You may be able to get a good price on a bundle of the tank and necessary equipment. (See Things You'll Need below).
Plan on total setup taking a period of a full day or two, and don't plan on stuffing your new aquarium full of fish right away. The key is to stock it very slowly.
Read How to Set up a Freshwater Aquarium to get an idea of what you're going to face once you get your tank.

Tips

Acrylic tanks are lightweight, shatterproof and have rounded corners that look a little bit nicer than the glass tanks, but they are considerably more expensive, and scratch very easily.
Glass tanks are much harder to scratch (but it can be done) and are less expensive than the acrylic. They are also heavier.
There are some general guidelines for the number of fish you can comfortably put in a tank. Overcrowding leads to illness and death in the fish, so be aware of your space limitations.
- Tropical freshwater tanks: 1 inch of fish (includes tail!) per gallon size of the tank. For example, in a 10-gallon tank you could have five 2" fish, or ten little 1" fish.
- Saltwater tanks: 1 inch of fish per 5 gallons. If quantity is your goal, this may not be the option for you.
- Coldwater (goldfish) tanks: 1 inch of fish per 2 gallons.
This information should start you off but you should also get some books on how to take care of your fish. Check at your local library. If you have freshwater fish I reccomend "Freshwater Aquriums - Basic Aquarium Setup & Maintenance - by David Alderton"

Warnings

Don't overcrowd your tank. The old saying that a fish won't outgrow its container isn't true, and crowded fish will get sick and die. Happy fish are healthy fish with plenty of space. Plan accordingly when choosing the tank of size.
DO NOT GET A USED TANK!!! Do not buy your fish tank secound-hand. The previous owner could of cleaned the inside of the tank with chemicals that can harm the fish.
DO NOT CLEAN THE INSIDE OF THE TANK WITH HOUSE-HOLD CLEANERS!!! Doing so can contaminate the water and kill your fish. Only use water or special chemicals made for cleaning a fish tank.