In the 1.13-pre3 assets/minecraft/textures/items folder, you can find the following types of buckets:

bucket
clownfish_bucket
cod_bucket
lava_bucket
milk_bucket
pufferfish_bucket
salmon_bucket
water_bucket

There is no tropical_fish_bucket, or even tropicalfish_bucket.

There are also items for the following fish:

clownfish
cod
cooked_cod
salmon
cooked_salmon
pufferfish

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.