* Create a website, get affiliate links setup for
selling fishtanks etc
* Create cool content people will stay around for
* Setup mailing list to further monetize etc
* Start spending money on adwords etc to get people to join
* Start optimizing to make sure you make more money than
you spend
That's only one way, but it's pretty sure to work if you stay on the ball.
I've been making a living out of advertising income for 10 years. I think if anything it's easier now to make money out of advertising. Back in the day you didn't even have adsense/adwords etc.
It would be fun to do such an experiment, but I'm way too busy and way too lazy to be the one to do it :)
I've been earning money from advertising for a decade as well (actually, 12 years to be more precise) and the landscape has changed quite a bit.
Making money once you have your community set up is only part of the problem, getting a working community off the ground is the hard part.
When adsense/adwords did not exist CPMs were much higher than they are today and such a venture would have been easier to get to profitability then it is because of that. Some costs (notably bandwidth) have declined, but simply slapping a forum on a server and calling it a day is not going to cut it. It will take a good bit of time to get people to re-visit your niche community on a daily basis.
And there are already to many fish-tank forums that you'd be competing for a fairly small slice of the cake.