3) The mechanical filteration keeps the water clear.
I was wondering about this. I had read somewhere that even the though filters like power filters clear the water they don't really clean the water, because it just keeps flowing through the muck they have collected. This makes sense to me. Does anyone know if this is true?
On one hand--I use filters on most my tanks. I'm more of a fish person so I need it. The reason I got into plants is because of how much better it is for the fish and how much prettier they make the tank.
On the other hand--you'd be surprised how many fish you can keep healthy in a well planted, non filtered tank. Last summer I had a 5 inch goldfish and about 15 minnows and a 30g outdoor, no filter "pond", completely filled with elodea. All the fish thrived.
So, I think if you do it right you don't need a filter, but I still prefer to use them, because it is easier.