Let's say you had an aquarium with fish in it. By some miracle of physics it flash froze. Then by another miracle it flash-melted. Would the fish survive?

Might.
When we had the land with a pond and it flooded in January. Rick suddenly noticed all the small fish fry in the pasture surrounding the pond. They were stranded there for a couple days after the water receded….they were all encased in ice.
Rick started picking them up and putting them back in the pond….they were alive! There were a couple.hundred of them. Had grandkids helping. It was really cool. All those frozen fish scattered in the grass of the pasture. The warmth of our hands was enough to bring them back.

Probably not. The water in the gills would expand upon freezing possibly destroying the fish’s ability to attain oxygen from the water. Not to say what could happen to it’s heart and other organs…
This assumes that anything will freeze solid, not just partially or not completely.

There are frogs that freeze solid all winter until spring.

The fish would die.
Dutch. There are a handful of species of animals, that can freeze solid. But they are very rare. Most use a certain type of glucose, that acts like anti-freeze (ethylene glycol.) Without it, the freezing process is fatal.