What to do?
You need to click/touch on the bubble to move it near the fish. You will find that it will disappear when it reaches near the fish. So, until it disappears, you need to click/touch on it without big intervals.

Keep it in mind:
if you fail to complete the task, you will not get score.
You have to wait until next bubble appears on the screen.
If you don't see a bubble on your screen for more than 1 minute then restart application.