Canister filters are some of the most popular freshwater filters in the hobby. Without taking up space in the tank they provide a large filter volume. They are excellent garbage collectors and aerobic bio filters. There is one problem with this however. They are nutrient factories. Nitrate, phosphate and other bacterial waste products go into the water column. This turns the tank into a very nutrient rich place. As an example freshwater aquarists frequently report nitrate concentrations of 50mg/l. "So what?" you may ask. If you do then this filter is probably not something you need. High nutrient levels in the water can cause great blooms of fast growing algae which make the tank look ugly and "unnatural". But this is easy to prevent by reducing the amount of light and using efficient algae eating fish to hold down the growth. Another method is to add much light, but let fast growing plants compete with the algae for nutrition. Here too, the plants are supported by algae eating fish that keep the algae down.
I am in a different situation though. I keep native north European freshwater fish in a biotope aquarium. There are no algae eating fish or other efficient algae eaters. The plants are mostly slow growing and seem adapted to nutrient poor environments. In a nutrient and light rich environment they get overgrown by algae very quickly. But this is not the only problem with traditional filtration. A tank with so much waste products being processed will have large concentrations of bacteria and other micro organisms in the water column. This is a problem for some of the northern fish species. I have done almost clinical tests of this with European brown trout (Salmo trutta). I kept several batches of trout in a small aquarium under similar conditions for each attempt, except for one thing, a UV filter. The trout consequently caught diseases in the small and warm tank. But with a UV filter it took much longer time before the illness could be seen. I concluded that the organisms that were killed by the UV filter, bacteria and maybe some other micro organisms in the water column, played a major role in the disease. So, there are several reasons why I need to get rid of as much of the detritus as possible before it becomes food for micro organisms. I must try to create a cleaner and more nutrient poor environment.
A typical canister filter is a vertically standing container with water flowing in at the bottom going up through the filter medium.
The bottom chamber is a very important part that tends to be overlooked in discussions on the net. To see why it is important, try to look at the canister as a pipe. Then look at the cross section area of this pipe compared to that of the hose that goes to the tank. My Eheim filter has an area of 36100 mm2 while the hose has an area of 210 mm2. If water flows evenly through the whole system it flows 172 times slower through the canister than through the hose. Of course this figure is reduced by the density of the filter medium, but it shows the point. Water flows slowly up through the canister. Since the lifting capacity of the water depends on its speed it is reduced to only a fraction of the lifting capacity in the hose and gravity helps reducing it even more. But this requires even flow. Over the bottom chamber a medium consisting of ceramic tubes is placed. The tubes create a turbulence that creates friction and this friction again causes a general resistance against the flow. In combination with the bottom chamber this distributes the flow evenly up through the filter. In fact the speed is reduced so much that many large particles stay in the bottom chamber never entering the filter medium at all.
A coarse sponge and filter wool are also used for mechanical filtration. The main portion of the filter is filled with gravel, however. The gravel provides efficient bio filtration because its large surface area lets bacteria grow and get access to nutrients in the water. But the waste products from the bacteria go into the water column.
My idea is to remove the whole bio filter and improve the bottom chamber by expanding it so that it can can use the "vortex" effect that is commonly used in pond filters. The water is sucked into a round chamber in such a way that it spins around a few times before flowing up. This creates a permanent vortex in the chamber. Large particles have a tendency to drift to the middle of the vortex and sink down.
The backwashing complicates things a little. 2 new openings in the filter must be made giving a total of 4. Valves must be installed on all of them. The first, the wash water opening, is on the top next to the pump. Here a hose from the tap is connected. Note that the tap water is under heavy pressure so the canister should be strong enough to take this. Otherwise one must be very careful and make sure that the bottom valve is open when the wash valve is opened. The bottom exit is plumbed to the sink.
The wash cycle goes like this:
The fine filter wool will probably not get washed well enough. It must be changed manually. It can be omitted, but I would use it because it helps so much in keeping the water clear. The particles it stops are too small to be stopped by slow flow and coarse mechanical filters. So it adds extra value to the filter.
The coarse sponge is placed under the ceramic tubes to keep as much dirt sirt as possible from getting stuck in them. But it could be that it's best placed above the tubes, both for practical and filtration reasons. If it is placed under, the tubes may squeeze the sponge flat so an additional mesh plate is needed to hold them up. Also, it could turn out that particles are harder to wash out of the sponge that the ceramic tubes.