Fish, dolphins and squid stop spawning in the ocean biomes after placing ~16 Bucket of Tropical Fish in a body of water. Drowns and turtles appear to be spawning though. I also tested this in the latest snapshot (18w31a) and did the same thing. Tested in a newly generated world on version 13.1 and the snapshot as well.

I noticed this on my main world that was on versions 1.12.2 update to 13.1. Attached is the log file and screenshots.