I think the question might be better phrased as "what will end up being my filter if I don't use a hang-on back/sump?"
It's worth keeping in mind that even in a stagnant system with no water movement you'll naturally get sedimentation and settling of particulates on to your sand bed given time. This is why most people go with mechanical filtration. As supposed to having particulates settle on your sand bed and having that be your filter basin we can make use of filter media to 'catch' particulates and then remove the media every so often to remove it from the system.
I hope this helps you a little bit at the least! Happy reefing