Hang-on filters, also known as backpack filters, are an interesting alternative to the more widespread internal and external filters.
Like an external filter, they do not require any space in the tank and there are hardly any visually disturbing elements. Like the internal filter, they are simple in design, easy to maintain and leak-proof.
The catch? There is a thick knob hanging on the outside of the tank. If the aquarium has enough space to the wall, you can hide it well. Otherwise you have to know where to set your priorities. We would also be happy to sell you an internal filter ;)