When you insert more layers those also need to protrude above those that are in front but remain smaller than those behind them.
By creating many layers we can give a layout a powerful sense of depth. The layers further towards the back must not be covered entirely by those lying in front of them. Each layer needs to be at last partially visible to the onlooker, i. e. the layers in the rear need to exceed those farther towards the front in height. When you insert more layers those also need to protrude above those that are in front but remain smaller than those behind them. Theoretically, this effect can be continued infinitely, however, sooner or later you will run into a physical border: the back pane of your tank. The space in an aquarium is limited, after all, and for this reason it needs to be used as efficiently as possible.