Good fish that this works on:
Airsharks
Stone Coatls
Dolphins
Gwees
Now, before the game loads, tap the center of the tank as much as possible to get the fish to the center.

Wait for the countdown...

If done right, the fish (or most of them) will be frozen in the center. This makes for easy grabbing.

Now, a lot of you may have known about this, but most likely you didn't know that during the game you can keep the fish frozen by tapping the blue bar at the bottom of the tank.

Note: When doing this, Airsharks and Stone Coatls will likely max out.
Note: On Dolphin tanks, it works best if you tap the blue bar in between when the red gold falls. (That way if you miss some gold, you're missing regular gold. Not Gold worth 5 each.)