Help with simple game (moving fish)

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@sara

And how do I make the amount of seconds random?

For time we can use millis() or frameCount:

millis() / Reference

frameCount / Reference

If you pick frameCount, remember that draw() is called back around 60 FPS.

So, if you wish for your fish to turn around between 5 to 10 seconds you could use this:
nextFrameChange = frameCount + (int) random(5 * 60, 10 * 60);

Then keep polling the value of frameCount until it reaches nextFrameChange:
if (frameCount >= nextFrameChange) swapDirection();