Resolution: Works As Intended
In the 1.13-pre3 assets/minecraft/textures/items folder, you can find the following types of buckets:
There is no tropical_fish_bucket, or even tropicalfish_bucket.
There are also items for the following fish:
Again, tropical is not shown.
When playing in the snapshot, when I pick up a tropical fish, it shows the clownfish_bucket graphic with the tropical fish bucket tag.
If I pick up a clownfish (with a completely different bucket), it also shows the clownfish_bucket graphic with the tropical fish bucket tag.
Something's goofy somewhere. I've tried going through the .json files in the different areas, and I just don't know enough about all of that to even remotely follow where the chain of events must have gotten broken, somewhere, but it's definitely broken.
I'm pretty sure that I got a regular clownfish bucket in 1.13-pre1, but I'll have to test that...don't have time to do it tonight.