Placing the Display
Craft or obtain a 
Fish Display block and place it where you want to show off your catch. It functions as a mount for a single fish item, similar to how an item frame holds an item but themed for fishing trophies.
Mounting a Fish
Hold a caught fish item in your hand and interact with the placed Fish Display. The fish is inserted and rendered on the block, letting visitors see the exact species you caught. To remove or swap it, interact again while empty-handed or with a different fish to replace the current one.
Building a Trophy Wall
Line several Fish Displays along a wall to create an aquarium-style showcase of rare catches like the 
Golden Snook, 
Demon Gar, or 
Crystalline Snakehead. Because each display holds one fish, arrange them in a grid for a collection wall. This is the intended way to show off completion progress without keeping fish loose in chests. Combine with labeled signs if you want to name each species for other players on a server.