All you have to do is to tap on the fish with the same colour(s) proposed on the menu above.
This would be a free mode play.
Keep an eye on the countdown.
But if you want a challenge, go for story mode and at each float one task is required.
It will change the difficult as you make progress in the levels.
For now, 15 mind sharpener stages, reaching your brain comprehension to make it to the end.
Take a moment and go fish!