The poem is "Fish out of Water" by Louis Jenkins.
Here’s the actual poem:

When he finally landed the fish it seemed so strange, so unlike other fishes he’d caught, so much bigger, more silvery, more important, that he half expected it to talk, to grant his wishes if he returned it to the water. But the fish said nothing, made no pleas, gave no promises. His fishing partner said, "Nice fish. You ought to have it mounted." Other people who saw it said the same thing, "Nice fish. . . ." So he took it to the Taxidermy shop but when it came back it didn’t look quite the same. Still, it was an impressive trophy. Mounted on a big board the way it was, it was too big to fit in the car. In those days he could fit everything he owned into the back of his Volkswagen but the fish changed all that. After he married, a year or so later, nothing would fit in the car. He got a bigger car. Then a new job, children. . . . The fish moved with them from house to house, state to state. All that moving around took its toll on the fish, it began to look worn, a fin was broken off. It went into the attic of the new house. Just before the divorce became final, when he was moving to an apartment, his wife said "Take your goddamn fish." He hung the fish on the wall before he unpacked anything else. The fish seemed huge, too big for this little apartment. Boy, it was big. He couldn’t imagine he’d ever caught a fish that big.

My interpretation:
it’s hard to recognize a good thing before it’s gone.

The fish wasn’t really the focus of the story… sort of a prop representing the moral of the story.

The poem seems to be about regret. Having swam so far upstream only to be pulled back downstream. It tells a story of distractions from what’s really the most important. How a large thing (a problem, for example) can get in the way of the things which should be paid more attention to.