That's not how supply and demand works.
Let's not look at the macro economics part of your statement about income and prices. That's too much for a message board.
But the economy is doing very well, causing more people to have disposable income and scuba. So just looking at supply and demand....
As demand for tanks increases, the demand curve shifts to the right, increasing price and increasing quantity demanded. More people doing the hobby actually increases prices, assuming the supply stays the same. And I don't think Luxfer or any other tank manufacturers have had big changes to the production process/quantity in YEARS.