You should be aware that purchasing a fish tank with 100 gallons is not a cheap choice if you are an experienced aquarist considering upgrading to a giant aquarium of this size. A tank that holds 100 gallons would need more maintenance and more sophisticated tools. A regular glass tank in the shape of a rectangle may cost you anywhere from $150 to $3,000. Starting with a starter kit for a smaller tank may be a good idea if this is your first time keeping fish in an aquarium.