About filtration

The topic of removing a filter is very interesting. The removing of
floating detrius and water circulation is what is meant by mecanical
filtration, no? But isn't it true that the nitrifying bacteria live in the
filter and on the sides of the tank and in the gravel -- actually on all
the surfaces? In other words, biological filtration takes place to some
degree no matter what.

I'm interested in the idea of minimal filtration. What kind of filter
system would be recommended by those who propose minimal filtration? On my
20 gallon tank at work I have an Aquaclear 150 filter. The sponge in it
catches dirt in the water but also acts as a biological filter by giving a
home to the bacteria. What could one replace it with?

in Vancouver which is very un-May-like right now.