Choosing a Canister Filter
Of all the methods of filtration available for the planted freshwater tank, the canister filter has long been the superior choice. Located beside or underneath the aquarium with only intake and outlet hardware inside the tank, the canister filter is arguably the easiest type to hide. This is of primary importance in the modern planted aquarium, where a natural look is paramount to the design of the system.
Beyond the aesthetic considerations, canister filters are versatile, in that all brands offer the option ofusing a variety of media inside the canister. Canister filters are also efficient, because they operate under pressure. This ensures that whatever media are inside will be uniformly exposed to the water flowing through the unit.
Perhaps the sole disadvantage of canister filters is the requirement for regular cleaning and maintenance, which in turn necessitates disassembly of the entire filter. While no filter design can eliminate this requirement, thoughtful attention to the design can mean the difference between maintenance being a nightmarish chore versus a simple, quick and easy procedure.
Factors in Filtration
Three types of filtration can occur in all canister filters. These are mechanical filtration, chemical filtration, and biological filtration. The use of chemical media for specific water purification tasks is typically determined by the dictates of each hobbyist’s particular circumstances. Chemical media can be generalized, such as activated carbon, or more specialized, such as media that absorb ammonia or phosphate. Biological filtration occurs in all filtration systems. It consists in the conversion of ammonia to nitrate by beneficial bacteria in the presence of oxygen.
These bacteria cover every surface in the filtration system, wherever sufficient oxygen is present. So long as the oxygen is available, the efficiency of biological filtration increases as the flow of water throughthe system increases. Nevertheless, the maximum practical flow rate that can be used in an aquarium system exhibiting plants and fish is likely to be lower than the ideal maximum flow rate that could be used, simply because one does not want the tank to be churned like a whirlpool bath.
Mechanical filtration is the trapping or sequestration of particulate matter from the water in the aquarium. Variables that influence the efficiency of mechanical filtration include the nature of the filtration medium, flow rate, and efficiency of flow through the medium. Most systems employ a sequence of increasingly smaller pore size media. Some may also include biological filtration media, designed to maximize surface area for colonization by nitrifying bacteria. The media compartment must be designed to encourage water to evenly and fully penetrate the media, without dead spots.
Flow rate is frequently touted as the main criterion for matching a canister filter to a given aquarium. Here are two recommendations, easily found online:
“A good rule of thumb is to select a filter with a flow rate at least six times the volume of the tank—so for a 30-gallon tank you want a flow rate of about 200 gph (gallons per hour).”—Tropical Fish Hobbyist *
“We generally recommend that on average, you turn over your aquarium volume four times per hour. This means that if you have a 30 gallon tank, you need a pump and filter with flow rates of around 120 gph.”—LiveAquaria.com *
Obviously, there is no consensus on the optimal flow rate.
The topography of the aquarium aquascape will have a large effect on the turbulence produced at a given flow rate. Further, for heavily planted aquariums, too much surface turbulence can drive off carbon dioxide, to the detriment of plant growth. For this reason, any change in filtration system for a planted tank should be followed up with testing to insure that the carbon dioxide concentration remains high enough to support vigorous plant growth, typically about 60 ppm.
Given the wide variation among filtration systems and filter media, it is simply not possible to define a given flow rate or turnover rate that is optimal in all instances.
While it is true that aquaculture research has demonstrated that ammonia conversion efficiency increase with increasing flow rate, so long as sufficient oxygen remains available, those experiments were carried out in ponds and at far lower turnover rate, i.e., one turnover per hour, than most aquarium systems employ.
No single factor can be isolated and defined as “optimal” for a given aquarium filtration system. The goal should be to install a system that effectively removes particulate matter and achieves adequate biological filtration, while at the same time avoiding excessive turbulence that will either rob plants of carbon dioxide, or create more current than the fish prefer.
Article written by John Tullock