It's not that it isn't cruel, but that it's *worth* the sacrifice. And I would disagree. Would you leave a fish out of water just to better understand the effects of no water on a fish? No, because you already know. Experiments on fish are subject to regulations just like experiments on other animals and are done under strict controls. The amount of fish and the way it is approached must be approved beforehand. I've read of experiments where individual blindfolds were made for fish rather than simply blinding the fish. Experiments on fish can be legitimately worth the sacrifice, but that is why a system was designed to determine that.
And, like I said before, puffers are fickle creatures. They can be fine with your fish for weeks, and then decide to attack them. As soon as one of those mollies tries to eat his food, he'll notice it.