[citation][nom]warmon6[/nom]what kind of fish your talking about that wont be happy in 6 inch deep? My brother has a 10 Gallon fish tank that only 5 inches(at the middle 8 inches due to the outward curve in the front) deep and the fish has been happy in there for quite a while.[/citation]
I don't know what kind of weird 10 gallon tank he has, but the normal one is thicker, being roughly 10 inches thick. The 10 gallon tank is lousy as well, for all but the smallest fish.
Unless you know fish, you probably can't tell the difference anyway. How would you know they're happy?
But, to answer your question, put any fish even an inch long in a tank, and he's going to want more than six inches of depth. Cichlids are probably the most interesting of the freshwater fish because of their intelligence and personalities, and they will go crazy in such a narrow tank. I always get the widest tanks possible, and this produces fish that breed more often, have more vivid colors, and act normally as opposed to neurotically. They are also much friendlier to humans and learn much faster when happy.
If you have the choice between, let's say, a 48"x13" tank (55 gallon), and a 36"x18" tank (50 gallon), the fish will prefer the latter, even though it's slightly smaller. For large cichlids, you want 24" wide tanks of at least 180 gallons. Anything else is a cruelty.
A lot depends on the personality of the fish. If you get characoids, which are generally a lot less intelligent, but have their own personality quirks, you give them what they like. For example, Cichlids are very territorial, and love to claim things like rocks, and caves, and such. So, you give them these things. Otherwise they get bored and are unhappy. Piranhas, a characin, love to swim against a current. If you put them in a tank without a current somewhere in it, you're not giving them a good life. They also have no love for rocks, and like an open clear environment. Pacus, their closely related cousins, also like open areas more than closed.
The point being, if you're going to place something in your hands, make sure you wash them good. You have a responsibility to them, and if you're smart, you can give them a good life and at the same time enjoy them. A device like this totally destroys any possibility of that, except for the smallest of fish. These are living creatures, not electronic devices. When you're willing to live in a closet, then put your fish in one of these. If not, give them something they can be happy in, and you'll be happier with them.