Basically this is just moving the fish within a tank while the multi tank panel is visible. This is done by tapping and dragging the fish. I like to use an upside down "u" gesture but any simple gesture should do as long as you end it below and to the side of the starting point. During the gesture, you can confirm the type of fish you are catching. You can then either push the fish to the tank position you want it to land in or lift your finger to release the fish without having the fish info display appear. This behavior use to irritate me but as I deal with large numbers of fish in a tank on a small screen, this seems to help.