My first tank was a cube. To be honest a cube is not the best shape for an aquarium. The surface area for oxygen exchange in a rectangular tank and the floor area for fish in the more conventional shape means you are at a slight disadvantage for stocking when compared to other 60 litre tanks. It does not matter work with what you have.
On the subject of changing filter cartridges. It may come as a surprise but filter sponge/ fabric or cartridges are not replaced for years. They are rinsed in old tank water. The bacteria live on all hard surfaces but mostly where there is a current of food ( poop) for them to eat.
Have you read of the nitrogen cycle?