From my experience with Fabio's Version - Fish spawning. It worked best for me if I (full tick[bold check]) 1 fish per category (river/sea) and all other fish entities are unticked. I haven't tried all fish just yet, but definitely the ones that I've tried were able to spawn, after you despawn the fish that are already in the water. Using bait also worked with spawning the ticked fishes.
Fabio's Critter cheats
There are 3 states that you need to consider
Unticked = no spawn,
Half Ticked (Grey tick) = normal spawn,
Full Tick (Black tick) = increase frequency
For best results, Untick everything, Solid tick 1 bug, 1 fish river/pond, 1 fish sea at a time
If you know which are river mouth or pier then try them at the same time, they are location specific.
Clear out existing bugs/fish on the island by catching, scaring off. The new ones you have forced (Full tick) should start to appear.
If you can't wait for spawning or just don't want to run around fish finding, use bait, particularly freshwater pond fish or out on the pier.
All bugs and fish caught are added to your Critterpedia and can be donated to the museum or sold in the shop.
The Poker modifies the time and dates when they can be caught to 24/7 and the frequency they appear.
The Coelacanth, can only be caught if it's raining
Some fish can only be caught at the highest point of your island. i.e. Cherry Salmon ,Char, Golden Trout etc
Useful links that tell you when and where bugs and fish normally appear.
https://www.ign.com/wikis/animal-cr...ice,_and_Fishing_Tips#Football_Fish_.E2.9C.93
https://www.ign.com/wikis/animal-cr...Bug_Catching_Tips#Common_Bluebottle_.E2.9C.93
Regards