The bug

If there are at least 16 fish spawned from buckets anywhere in loaded chunks (that might be the spawn chunks, chunks loaded via /chunk, or chunks that players currently are in), no aquatic mobs (fish, dolphins, and squid) will spawn anywhere.

How to reproduce

1. Create a new superflat world (I chose "Redstone Ready") and change it so that it has biome minecraft:ocean
2. Run

   /gamerule doMobSpawning false

3. Create a big aquarium at spawn

   /fill ~ ~-1 ~ ~20 ~-21 ~20 water

4. Drop 16 salmon from buckets in there
5. Teleport to somewhere far away, for example

   /tp 1000 56 1000

6. Check that all salmon still are there

   /execute if @e[type=salmon]

7. Create a big aquarium there as well

   /fill ~ ~-1 ~ ~20 ~-21 ~20 water

8. Run

   /gamerule doMobSpawning true

9. Notice that no mobs spawn there whatsoever
10. Run

    /kill @e[type=salmon,limit=1]

11. Notice that suddenly, there are many mobs spawning in that aquarium

How to fix this bug

Change the aquatic mob cap so that fish spawned from buckets no longer affect it.

Original report

As stated tropical fish and pufferfish are not spawning whatsoever in my main world. I created another world to see if they would spawn there and they spawned just fine. I have spent hours upon hours in my main world and I don't want to have to restart. Is there any way to fix this? I don't know if it's a factor, but I had to travel a fair distance from my spawn point to find warm ocean.