It is actually interesting - Fyi - in case anyone is curious the total spent on pet care in the US - is >75 billion/year
Starting to wonder if I found the same source as you.
Following along.
My initial guess of 300,000-500,000 households was guessing how many people have REEF tanks. The OP asked about marine tanks, which would include fish only tanks..I would have guessed closer to a 750,000+ figure for that..I should have read into the question a little more. I know a couple people who can keep fish only tanks, but struggle with any type of coral. I would also guess that nearly half of people who start reef tanks get out of the hobby within 2 years of starting because of difficulty, time, money and getting burned out. I would also guess that for every person that gets out of the hobby a new hobbyist replaces them at a pretty even rate.
I would be interested to hear what % of people who have marine tanks started out with freshwater. I cannot imagine that many people dive right into marine tanks, more probably advance in the hobby and transition to marine?
I've never kept a freshwater tank. My parents did when I was younger. All I did was feed the fish back then. Straight to saltwater for me