Why do the fishes fins stand up?

My fish recently started holding there fins on their backs up and im wondering what this means. Any help would be great thanks!

 

From my experience this is a territorial defense mode tangs especially.

 

oooo ok hope they dont start fighting. It happened after i turned a light on in the room so i thought it might be that.

 

If a fish gets startled this could happen as well. Kind of like Hackem said its defense mode since the turning on the light and seeing other things probably startled it.

 

My tang does that when startled. My wrasse does this at feeding time. Kind of like a cat who's fur is standing on end. It is basically to look bigger and badder.

 

Ooooor it could be like a dog. A dog whose tail stands up all of the time means he owns the place. Including the house. Or, my cocketiel's top knot sticks up when he's curious, thinking hard (this is rare...he's on the slow side), or startled in a "WHAT THE HECK WAS THAT" manner.

For fish it's territorial depending on the fish.

 

I see it alot when I introduce new fish to the tank so I think it is mostly territorial and a defense mechanism. When I first added my sleeper banded goby he would raise his fins everytime I got too close to the tank. Also some males will do it when mating to display their colors to a female.

 

ok i was worried for a little because i thought it might mean that the fish are stressed due to the water being unhealthy. What signs can i look for the might mean the fish is under stress? Thanks

 

so they look bigger. for better meanuvability while swimming.

 

I moved some rock around once and my blue hippo was MAD! She swam around with those fins straight up on and off for hours after that! For her to raise her fins is rare, I could tell she was very unhappy with me!

My foxface does it when he's swimming fast and comes to a quick stop, and sometimes to act big and bad to other fish.