Many generations.
Are your corals breeding sexually?
Not true. Rapid evolution happens all the time in nature. A forest in North America may start out with a bunch of squirrels and birds living in trees, but over the course of the squirrels' and birds' lives, some will evolve into monkeys, some will evolve into giraffes, and some will evolve into sharks, who subsequently evolve to have no gills so they can survive in the trees. Some squirrels will physiologically remain normal squirrels, but they'll be fourteen feet tall with big, 15" buck teeth. Some will also be so small, they look like ants. Some will evolve into inanimate objects like dirt and rocks. Some will evolve into water too.
This rapid single-lifetime evolution happens with domestic animals too. I adopted a puppy a few years ago, and while he started out a normal dog, he's now a small horse (15.1 hands). Also, he's now a she. The clownfish pair I bought a few years ago are evolving too. One is almost done evolving into a stomella snail while the other is about three quarters of the way through transforming into a hamster. I've built a hamster enclosure for him to live in since his gills are evolving away and soon he'll need to breathe air.
Humans are also really adaptable too. It's true we evolved over millions of years to breathe a very specific mix of nitrogen and oxygen. But it's not like we'd die if we don't breathe exactly that. Humans can really breathe any gas, like chlorine, HCl fumes, pure carbon monoxide, the exhaust from coal power plants, etc. It's not even really limited to air. Humans routinely breathe liquids like water, tomato soup, antifreeze, laundry detergent, etc. There really are no limits on anything ever.
If all the above is possible, why can't you put an animal that has evolved for millions of years to live in a very specific water chemistry into another? It would be one thing if just raising the temperature a tiny bit or tweaking pH down a touch kills millions of corals per year. But there's no such thing as coral reef bleaching. In fact, there's no such thing as anything ever.
Come on man, open your eyes mind and start thinking possible.