Sorry for continuing the highjacking of your thread, Fishman.
Where did you learn this information?
I know a gal that breeds betta in 10 gallon tanks with no issues. Some of hers are 5 yrs old and have lived their whole lives in 10 gallon aquariums.
How deep does your friend keep the water in the Betta tanks? If she is breeding, then it most likely is 6 inches or less.
Betta's have evolved over thousands of years to survive in water that is 2-6 inches deep. The swim bladder is a specialized organ that evolves with the fish to specific environments. If this is the environment the fish evolved in, any parameters outside of this will cause undue stress to the fish. You have to consider that the Betta is a highly specialized organism.
Water pressure at 6 inches depth is 6 x 0.03613 = .21678 psi
Water pressure at 1ft depth is 12 x 0.03613 = .433 psi
Could you live comfortably at 2x atmospheric pressure?
Placing a betta in a full 10g tank will expose him to water pressure double that of which he is physically designed to live in.
I have also been looking into this in regards to caring for Blue Rams because they also have evolved to live in shallow pools like the Betta and dropping them in an 18 or 20 inch deep tank might be the cause for prevelant swim bladder problems.