Minecraft wiki entry for Bucket_of_aquatic_mob, associated text: When a water bucket is transformed into a bucket of aquatic mob, it actually pushes the bucket texture down one pixel. According to Dinnerbone, JAPPA says this is due to the "weight of the fish".[2]
Buckets of tropical fish always appear to contain a clownfish, regardless of whatever fish is actually inside.[3]
Since empty buckets can be used to capture fish or axolotls in Bedrock Edition, it is possible to create an infinite amount of water source blocks simply by catching a fish mob with a bucket and continually placing and re-capturing the mob.
The bucket of cod and bucket of tropical fish are actually re-colorations of each other.