How do I make objects spawn in a grid?

Hi! I'm new to using Construct 3, right now I am trying to create a feature in my game where I have a white square on the screen, and when I click the spacebar a small fish appears on the white square. If there is already a fish on the white square then a new fish should appear next to it, and if a row is filled on the white square then the new fish should appear below it to create a new row. So at the end I have a grid of fish on the white square.

How would I go about doing this? Any help would be appreciated!

I would make it so every time a fish spawns, the square would move right by the grid size. If offscreen, or reaches a certain x, it should go back the the original position and go down by the grid size as well.