I am quite aware of the difference between evolution and an experiment. My point was simply that it theoretically is not a lost cause if given enough time. It takes time and numerous factors to evolve. Certain, quick reproducing, organisms can be found evolving within a very short amount of time, usually nothing game changing, but enough to adapt. Zoas reproduce fast enough where evolution can start to be seen rather quickly.
The op isn’t just putting them directly into fresh water, he is taking time to acclimate slightly over the course of many years, and has said he plans to stop the experiment if it starts going wrong. I think he knows that the chances of success was low, but that is part of what science is. You have to push the boundaries despite what those around you thought. We wouldn’t have any of modern science if people have up because their hypothesis went against the thought of the time.
If it is the corals’ lives you are worried about, a good portion of biological science revolves around animal testing. Just like how cows are bread for steak, these corals are being bred for the purpose of science.