Sir - First, I'm very sorry for the loss of your friends. I looked up the NTSB preliminary report on this accident and found it here: http://www.ntsb.gov/aviationquery/brief.aspx?ev_id=20120607X54234&key=1
The final word will have to await the NTSB Probable Cause report. For now though, I've got to tell you this reads like an airframe over-stress accident due to flight into convective weather activity. As such, the weather is not the cause, it is the flight entering the weather that would be the cause. There was quite a bit of discussion with ATC about avoiding the weather, but in the end, it looks like the aircraft was flown too close to a T-storm.
As for your question about a diving tank bursting, being a diver myself, I believe this unlikely. Tanks are deisgned to withstand overpressure by a significant margin, and we only have 1 atmosphere of air pressure change from the surface to outer space. Thus, being a pressurized aircraft, cleared to 26,000' MSL, the cabin would have been pressurized to well less than 10,000ft, which would have decreased the pressure on the outside of the cylinder(s) by about 4.6psi - hardly enough to cause a rupture of a tank. Apart from this, the above report states portions of the wings, elevator, and horizontal stabilizer were found separate from the wreckage. A tank rupture would likely have caused other parts to separate too.
I hope this helps your understanding.