An important aspect of filter redundancy is back up bio filtration, which is also useful for seeding new aquariums or quarantine/hospital aquariums, added mechanical filtration, added water movement, and preventing filter emergencies (especially while you are sleeping, out of town, at work, etc.), since your second filter takes care of filtration until you can fix or replace the failed filter.
Further Reference: .
Aquarium Systems Maxi-Jet Pre-Filter Sponges - 2 pk - Big Al's Pets
For Hydro Pond Filters, follow this link
Hydro Pond Filters are essentially extra large Filter Max Pre-Filters
Excellent for use in High Flow Aquarium Sumps, Central Systems or similar.
Can I Clean My Pre Filter Sponge? | My Aquarium Club
Occasionally, even with all the variations available with the three adapters plus the elbows provided with the Filter Max Pre-Filter, the filter does not fit as snugly as desired.
The above picture displays a SunSun HBL-501 Hang On Aquarium Filter where some Teflon Tape was added to make the fit more "snug".
Sweet Deal on Eheim Aquarium Pre-Filter - Better Homes and Gardens
This type of filtration is by far the most important for keeping fish in an aquarium. From the moment fish are introduced into the aquarium, they begin releasing ammonia, their principle waste by product. If allowed to build, this ammonia can kill fish by burning the gill tissues and preventing them from taking in oxygen. Beneficial bacteria help us filter the water through a biological oxidation process. Essentially, waste laden water passes over bacteria, which consume the waste and convert it into less toxic compounds. This process is called the “Nitrogen Cycle” and it is a LIVE filter. The Black Beauty fish tank prefilter gets hooked on before the Red Torpedo canister aquarium filter and accumulates all the nasty sludge, excess food and fish waste making this aquarium canister pre filter a must have for all aquarium keepers.Power Filters
Power and canister filters push or pull water through a set of media in a container with a motorized pump. Water is pulled into the unit through an uptake tube and passes through various media before being expelled to the aquarium again. Hang on the back power filters are small and inexpensive. Canister filters are sealed, pressurized and reside beneath the aquarium, inside the aquarium stand. Additionally, the powerful water movement allows these filters to be used on large tanks. With quick disconnecting hoses, it is easy to perform maintenance by taking the filter to the sink, so that water is not spilled. By using the Black Beauty fish tank pre filter you will be keeping your aquarium dazzling while ingeniously extending the life of your aquarium filter media.