We are getting ready for a reunion out of town so I cannot do it this weekend. I will try to do this next weekend for you. My tank have a painted back so I cannot put the mirror in the back, but this is easiest I would think. You will get picture of the reflection also.
You can put the mirror on the side, but the orientation of the fish is different and the fish will be moving fast toward or away from the camera.
I get the best picture if I put the mirror in front of the camera . Disable auto focus and focus it at a specific place just outside the edge of the mirror. As the fish flashes, and move forward, he will move past the mirror, still flashing. You only have a fraction of a second to snap the picture before he stop flashing.
If you object is one of the bolder wrasse, he will flashes all the time, and it is easy. Just make sure you have bright light, open the aperture and shorten the shutter speed for clear picture.
The last few pictures, the best pictures, of my Eightline Flasher were just pictures of him strutting around the tank in late evening, no mirror or gimmick. I just have the camera on correct setting and snapped a few picture. That was just pure luck. I keep my camera on a stand next to the tank.
Best of luck with your shooting. Hope you get trophies of your wrasses.
I have an Olympus E-M5II. The lens I like to use the most is the Olympus 12-40mm 1:2.8 PRO. Due to sensor element size, this lens is equal to 12-80mm Zoom in a 35 mm sensor SLR