I've used Shaper3D on my iPad, but it's not something that a total beginner would be able to pick up and start designing complex things immediately, but its more user friendly than CAD. Not designed for aquariums specifically, but it's great with layering and building things out to scale. There is a free version, but unfortunately I cannot recall what features are limited. It is designed around use with the Apple Pencil, so something to keep in mind..
With this in mind, I have personally found that the most effective method of mapping things out is with good old paper and pencil. You don't need to be an artist to make good plans, just a couple of pieces of paper, a pencil, and a ruler if you're feeling spicy.