Not with many fish, although holding a mirror to a betta will cause it to flare its gills as it will think its reflection is competition
The betta fish will see the mirror and then will be curious about it, because the mirror reflects the betta fish and they might think that it is another fish, so the fish will act differently.
it depends what fish u have if u have a gold fish it would have change their behavior and it will think its another fish
ebay.com
yeah mate!!
It will think that there is another fish really there and it will fight at it until you take the mirror away from it or it dies.
Placing a small mirror glass in front of your fighter fish will do the job
Put a mirror in front of it, that usually drives them mad.
put a mirror in front of the tank if the fish bloats out its gills its a boy.
A fish eye mirror is a convex mirror
MIRROR
A male Betta should flash and square up to a mirror image of itself if it sees one occasionally. If the mirror is presented to the fish too often or permanently, the fish will simply learn to ignore it.
You can place a mirror in front of it and it will fight as though it was another betta. However if you keep the mirror in for too long your betta will get stressed out and die, so best not to do that.