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Date & Time: All days, all times, since introduction in patch 6.3
Frequency: Permanent
World name: All worlds
Character name: All characters

Steps:
0. Reasonably fill out your Fish Guide by obtaining many fishes
1. Open Fish Guide
2. Select search function
3. Search for common fishes

Observation: Searching for terms such as "shrimp" or "loach" or "coral" will return 0 results, despite knowing there should be abundant representative fish.
Searching for "squid" managed to return 1 result, Megasquid. Similarly, "shark" returns few results
I consider, and try again, searching now for "Shrimp", "Loach", and "Coral" and find all expected results appear. Searching for "Squid" and "Shark" similarly returns all additional results

Conclusion: The search feature has been implemented to require expected capitalization of fish names. While this does technically work as intended, no other search feature in the game has this case sensitive behavior, so this function within the Fish Guide is presumed to be an unintended bug.