Minecraft wiki entry for Bucket_of_aquatic_mob, associated text:  BucketVariantTag: The variant data of the tropical fish in the bucket. Applies only to Bucket of Tropical Fish.
 EntityTag: Stores entity data that is applied to the aquatic mob when it is poured out. If this data includes a tropical fish variant, it overrides the one provided in BucketVariantTag.
See entity format.