Water takes the easiest path. In a canister packed loosely, it will run along the wall of the body and through the gap where the trays meet, so the last stage does almost no work while the first stage clogs in a fortnight.
The order that behaves predictably
The order that behaves predictably is coarse to fine, bottom to top: a sponge or wool pad first to catch the solids, biological media in the middle where flow is even and oxygen is still available, and any chemical stage such as carbon last, so it is not blinded by particles.
- Mechanical stage at the inlet — sponge or wool, the part that is rinsed and replaced.
- Biological stage in the middle — rings, spheres or sintered media, kept loose.
- Chemical stage, if used, at the outlet — carbon or resin, changed on a schedule rather than on appearance.
Reversing the first two is the most common mistake in a refill kit. Wool above the biological media catches nothing, because the media has already taken the particles out of suspension, and the wool then blinds itself against the top of the tray.
Pack it, do not cram it
Pack the trays so the media touches the walls without being compressed. Rings should be loose enough that a finger can move them; squeezed media crushes the pores that do the biological work. If a tray has a bypass slot, block it before adding media rather than after.
The same rule applies to the gap between trays. A tray that sits on a stack of media instead of on its own shoulder leaves a ring of open water around the outside, and that ring is the channel that undoes the packing order above it.
What this means for the range you stock
If the media is sold separately, the packaging has to carry the order, because the instruction is rarely read twice. A three-part kit sold as “stage 1, 2, 3” with the trays colour-coded cuts the number of support questions a dealer has to answer.