Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Think about it in terms of the water flow.
1) Coming from the tank the water is the dirtiest. Might have chunks of larger debris.
The first media should be quite coarse, something that will stop leaves (or parts of leaves) from getting any farther into the sump. Often the water from the tank is spread out over this like a spray bar, too. This makes the first location a conflict:
Coarse media to screen out leaves? Or Wet/dry, and use a bacteria media like Poret?
2) Medium then finer sponge to trap finer and finer debris. This is your main mechanical media area.
3) Chemical media, if needed. Peat moss, coral sand, activated carbon, Purigen and others. These materials are after the mechanical media so they are not clogged with debris.
4) Bio. Again, after the mechanical media. I would not use the plastic bio balls designed for pond use. Way too small a surface area for bacteria. There are many other bio media options, and most have a lot more surface area. As long as the mechanical media is doing its job to keep the water clean these finer materials will not clog up.
5) Fine mechanical right before the pump, just to make sure. For example, Purigen or peat moss might escape from the chem. section, and this fine floss would keep it out of the tank.