Minecraft wiki entry for Bucket_of_aquatic_mob, associated text: Pressing use with a bucket of aquatic mob places a water source block, and spawns the corresponding fish or axolotl back into the world, leaving an empty bucket in the player's inventory. For buckets of tropical fish, the fish and its details (color, size, name) are kept. Placing a bucket of aquatic mob in the Nether causes the same particles to appear that generate when a normal water bucket is placed in the Nether, and the fish is spawned alone.