Mechainical filtration would be ... if all you where keeping is an aquarium and fish..lol![]()
but you are not .. you are keeping a whole Ecosystem in a tank..
you are looking to simulate the ocean in your tank.. all a reef tank need to filter its self is..
Lighting, waterflow, Live rock and live sand!
Now when you take things away you need to add things in..
Most people will not or cant afford enough live rock! so then they will subliement the liverock with mechanical filtration.. but then they have issues with Nitrates.. But the issues are that they are never cleaned enough.. thats why you get nitrates.. but if you keep mechanical filtration and clean it correctly and fequently.. then you will not have issues!