Unless you replace (i.e., throw away) the mechanical media every month, your so-called mechanical filter will actually be partially a biological one as well.
So what you're really talking about is optimising filters for either mechanical or biological filtration. Your biological filter will HAVE to have some mechanical media, or the biological media (e.g., ceramic noodles) will get clogged with silt. So at the very least there will be some filter floss in there. The other filter can just be stuffed with filter floss, coarse ceramic noodles, or a whatever biological media you prefer. Once a month you'll need to open the filter, replace the floss and rinse the ceramic noodles.
In practise there's little mileage in going through all this rigamarole. Most folks find it easier to just use all their canister filters in much the same way, and rely on each filter dividing the workload. The exception to this is where a canister filter is used to drive a reverse-flow undergravel filter. Such filters are IDEAL for catfish tanks, by the way, because they force silt and faeces into the water column where the canister filter can suck them up. Clean water (i.e., mechanically filtered water) is pumped upwards through the undergravel filter (i.e., the gravel) and that's where the biological filtration takes place. Because the canister is only doing mechanical filtration, it's easy to maintain it without worrying about bacteria. The gravel itself stays very clean because of the rising water current.
Reverse-flow undergravel filters aren't much seen today in freshwater systems because they're not compatible with plants or huge amounts of rockwork. But if that's not a factor for you, it's BY FAR the best low-tech, low-cost filter out there.
Cheers, Neale
I am toying with the idea of using one of my three xp3 filters on my 90 gallon freshwater tank as a mechanical only filter. I was wondering what i should put in the baskets instead of the usual bio and chem media. Is this a good idea? and if so any suggestions on media and setup.
