Will a fish notice when another fish is no longer there?

[ElizabethFishton]

So one of my clownfish(Nemo) is swimming around the spot where my Strawberry Psetochromis, Purple Dottyback(Shiny)'s territory, Shiny jumped, we aren't really sure how and are actively trying to help shiny but the thing about Nemo is he only hangs out at the top, Shiny's territory I guess is kinda close to the top of the tank, and he's just swimming above it and actually going down as if to check if Shiny is there, so I'm wondering will they notice when a fish is gone, my other fish seem to be starting to do the same and see where Shiny is.

 

[KrisReef]

If a fish that is holding a territory in a tank leaves the other fish will certainly notice that the territory is empty and they move in and set up shop. They could be on the look-out wondering if Shiny is going to return because territorial disputes can be hard fought and are usually best to avoid.

 

[ElizabethFishton]

If a fish that is holding a territory in a tank leaves the other fish will certainly notice that the territory is empty and they move in and set up shop. They could be on the look-out wondering if Shiny is going to return because territorial disputes can be hard fought and are usually best to avoid.

Looks like Shiny's old best friend Domino the Domino fish has mostly taken over Shiny's territory, Nemo is still investigating though and hanging around the spot so maybe Domino isn't as protective as Shiny was