Jones... a 20 gallon long is 30" long. A 6" fish will have 5x their body length to swim. Lets compare that to the average accepted minimum tank for an oscar: 75 gallon, which is 48" long. An oscar can grow up to 16" long, but a good average size would be 13", meaning the oscar would have under 4x the length of its body to swim, yet this is still an acceptable minimum. I am currently keeping a 12" oscar in a 100g (just a taller, slightly wider 75g), and he is fine, even with some tankmates.
On top of that, I have kept 6" convicts in 20 long tanks. Convicts are thicker bodied fish than red tailed sharks, and are fairly active swimmers. They had plenty of room.
So please in the future keep your sarcastic comments to your self. There are many different ways to keep fish, and there are some people who wont put anything over 4" in smaller than a 55 gallon tank. That is their personal style. But from bountiful personal experience, a 6" fish will be fine and dandy in a 30" tank.
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