Tip: You can use Lunar Magic's Overworld Layer 3 Editing system to edit or create your own layer 3 images (castle bricks, fish, rocks) and apply unique effects per level via the Layer 3 Bypass in the main editor.
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Details for Per-level Layer 3 Tides Act-like Settings
This can be confusing to use at first, but it's a very useful patch for all kinds of gimmicks. To customize it, you'll need to find the part of the ASM file which says "levels:" (it should be right after the "freedata" directive). Each level number in there corresponds to a Map16 number in the "acts:" table right below. You can add as many levels as you want.
During the moderation, I included the levelnum patch within this patch. The custom code expects the level number to be stored to $010B, which is not a vanilla feature. Seeing as the levelnum patch is inserted by UberASM and PIXI, two essential tools, I didn't deem this a rejection reason.