These following mobs cannot properly spawn in the Tropicraft dimension as intended:
Tropicraft's tropical fish, Tropicraft's sardines, Tropicraft's piranhas, and any default Minecraft water mobs (squids, tropical fish, cod, etc.)

Observed issue: The various fish mobs are intended to spawn within the oceans and rivers of Tropicraft, however are actually only spawning within a narrow band of elevation, within deep underground aquifers and player-made trenches

It appears that the cause behind the mod's fish mob spawning mechanics is that they seem follow the default Minecraft rules for fish spawning mechanics (fish can only spawn in a narrow Y range, between around Y=50-63, as that matches the natural sea level in the Overworld). The problem for the Tropicraft dimension's case is that Tropicraft's natural sea level generates far too high up (around Y=125).

Whenever I used fill commands to make a trench deep enough to reach Y=63, or go into spectator mode to find underground aquifers, Tropicraft's various fish mobs did spawn as expected.

This also explains why no default ocean mobs could spawn in the dimension as well, as I also could only find default squid, tropical fish, and cod spawning within these specific areas as well.

This issue is being experienced in the Minecraft 1.18.2 version of the mod.
Edit: This issue also affects 1.19.2 and presumably the 1.20 version of the mod.

Additional bugs I've also discovered around the same time, so these issues are all reported within the same place:

1. Bamboo doors and trapdoors share the same crafting recipe, so bamboo trapdoors are uncraftable.

2. Home Tree Dungeons are only generating with pig spawners; I have yet to see any spawners producing ashens, or any other possible Tropicraft monsters.

3. Seagulls and Tropibees are spawning underground, which doesn't seem to be intentional.