In the editor, in the Spawner object properties, you can set a value for the cooldown. It starts ticking after activation. If the spawner gets activated again, before the cooldown expires, then that activation gets ignored.
If this has nothing to do with any puzzle, or aesthetic, but just needs to replenish the fish, then just connect a timer to the spawner, and set it for twenty minutes, to an hour or so. There are other ways, of course. It could be scripted to count the steps that the party takes. It could be scripted to count the fish, and respawn when they are all missing, or are too few. It could be scripted to respawn when they leave the level (using the cooldown to prevent exploit).
If you have many spawners like this, it would be better to set the timer to call a script function that activates them all; even better if it activates only some of them, and/or in staggered delay, so that they don't all pop into view at once.
As for the campaign file, the game installs the files; you have them already. Make a new editor project. Call it what you like; or name it Grimrock2. Then open the project file in a text editor, and change the contents to this:
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-- This file has been generated by Dungeon Editor 2.2.4 dungeonName "Grimrock 2" author "Almost Human" description "The official campaign" dungeonFolder "assets/dungeons/grimrock2"
Then reload the project. You cannot save any changes to this project; it won't work.