GameAnalysis

You can play the game at www.fishy-flash-game.com

Fishy is an old flash game where you compete to become the biggest fish in the pond.

You move your fish using the arrow keys. Your fish has a little momentum to its movement.

Other fish are generated to come in from the side of the screen at various sizes and speeds. If you collide with a bigger fish, it's game over. You must collide with smaller fish to "eat" them and grow. Every time you eat a fish you become slightly larger which means you can eat more fish but also it becomes harder to dodge the bigger fish.

I've had a look at the tutorial that John based his demo here:

https://phaser.io/tutorials/making-your-first-phaser-3-game/part1

The final game has a character running and jumping around collecting stars and avoiding bouncing bombs.
You can play it here:

https://phaser.io/examples/v3/view/games/firstgame/part10

We can relate this to the phishy game we wish to create as follows:

The player character moves under player control using keyboard or mouse control.

The demo has two different non-player objects, the star and the bomb. 
A challenge for us is that we only have one enemy class where smaller fish are eaten and larger fish kill the player. A comparison takes place on collision.

The bombs and the stars both move and we want the other enemy fish to move as well. Their movement is simpler.
In our game enemy fish are generated constantly at the side of the screen