You know what? It kinda depends on the type of fish you plan on getting. Foe example, if your looking into getting barbs, sharks type of fish that like to swim back and forth, and back and forth, a long tank would be best. Discus, angels type of fish like the tall tanks where they can swim up and down, and up and down. They are not much for swimming back and forth, and back and forth. ( :
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Another consideration is if you plan on keeping plants and what kind. Tall tanks tend to be good for plants because of growth however lighting tends to be a concern. You will need a higher output of lighting to get to the bottom of a tall tank where as in a longer tank, that might not be neccessary.
And another consideration is maintainence. It is easier on a longer tank than a tall tank. At least you dont have to get in the tank to clean it (tall tank).
How about a good suggestion. Get a long, tall tank. *crazysmiley* *twirlysmiley*