a better alternative to having an npc sense things in an area?

I have some npcs that use invisible parts with touch functions to respond to a player or brick's presence. I have this set up with a fish npc, but even If I change custom properties, the fish sinks too fast. I want to use a script instead of an acual part to sense things in an area, but Im not sure how to do this. I figured it would be along the lines of making three values, each with a vector3 position, but Im not sure how to set this up. anyone know if this can be scripted in roblox?