It all depends on what you're keeping.
If you have messy fish, such as herbivorous species like scats, then solid wastes (faeces, bits of plant material) can be unsightly. The ideal combination here would be 75% biological filtration and 25% mechanical filtration. Do bear in mind that biological media (such as fine sponges) will do some mechanical filtration, and mechanical media (such as coarse sponges) will do some biological filtration. So you don't need to be too obsessed over precisely which media you use, so long as there's a reasonable amount of mechanical media that you can rinse under the tap without worrying about killing all the bacteria.
Conversely, if you've got relatively few fish but they're very sensitive to ammonia and nitrite, such as puffers, then biological media is the prime concern. The filter could justifiably be 100% biological media, and the mechanical filtration such media does will be ample for the amount of solid waste produced. Rinsing the biological media in buckets of aquarium water will rinse off the solid waste without harming the bacteria.
If you live in a soft water area, then setting aside some space in the filter for calcareous media, such as crushed coral or limestone rubble, makes sense. This would be a real key to success if you were keeping species very intolerant of pH changes and low carbonate hardness, such as mollies.
The media you almost certainly don't need are carbon and zeolite. Carbon doesn't really do that water changes don't do better, and zeolite is redundant if you have ample biological media.
So within these parameters, simply tweak the filter media as suits your preferences and budget. If all else fails, it's hard to go wrong with a selection of good quality sponges, or a mix of sponges for mechanical filtration and ceramic noodles for biological filtration.
Cheers, Neale
