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Now on to biological filters. Some of the basics are up-flow filters (water enters the bottom of filter then flows out the top), down-flow filter (water flows in the top and out the bottom) and horizontal filter (water flows in one end and out the other). All other filters are variations of these designs. 

In the next section, I will go into more detail on biological filters. However, I have one last thought to leave you with: water follows the path of least resistance. How does this apply to a filter? What it means is it can be difficult to force the water around each piece of filter media or through the media. And that little bit of information is what makes the difference between a good filter and a poor one. 


I left you with the thought that water will follow the path of least resistance. This is the key to the difference between a good filter and a bad filter. The reason this is so is because the water going through a filter will try to go around the filter media rather than go through it. There is more resistance in going through the media than bypassing it. This differentiates between an effective filter and one that doesn’t really work. The filter should be designed so that the water going into the filter comes in contact with all surfaces of the filter media rather than going around it. 

How do we accomplish this? Usually the problem is that there is too much space between where the media comes into contact with the sides of the filter. I’ve seen filters where the filter media has been put into nylon bags for easy removal. The problem with this is that unless the bag fits the filter container very tightly, water will simply go around the bag. No bacteria will grow on the inside of the bag under these conditions. This is true of any loose media. If it is contained by anything other than the walls of the filter container, the water will simply go around rather than through the filter media. 

Another frequently used media is filter mat. This media presents its own set of problems. The mat is a solid piece; therefore, the water is definitely going to go around rather than through it. Here are tips on how to force the water through the material. First, cut the mat larger than the container opening and then tightly pack the mat against the edge so it is harder for the water to go around it than through it. Another suggestion is to make a frame for the mat so that the frame fits tightly against the edges and holds the mat so the water can not get around the mat. Yet another method is to wrap the mat around the delivery pipe so that the water radiates out through the mat. 

When using foam cartridges for prefab filter units, think of the fact that the foam expands when wet thus sealing the edges. 

Besides concern about forcing water through the media, we have to consider channeling. Channeling is caused by sediment carried in the water settling out in the media thus, restricting the flow. This causes the water to flow elsewhere, killing the bacteria where the channeling has occurred. It could also be caused by a build-up of bacteria. The only way to correct the channeling is to physically remove the blockage. One way to accomplish this is to reverse the flow of water and flush the debris away. Another is to bubble enough air to dislodge the debris in the media. Or you can physically wash the media. Just be careful not to kill the bacteria in the media. With some enzymes you may be able to break down the debris through biological action. Most of these products are called sludge removers. If you choose to use one, make sure it is safe for fish and remember that it will remove oxygen from the water which could also hurt the fish. 

The next item we need to cover in good filter design is the water delivery system. The water should be distributed so that it is spread over the surface of the media before entering the media. This will provide the media with the best chance to receive the constant flow of water needed for bacteria colonization. Remember that the bacteria we are trying to grow require oxygen in order to grow. Therefore, if you can add air before the media this will help the bacteria colonies. 

Next we need to discuss sizing the filter for the flow rate of the water going through the filter. It takes time for bacteria to break down the toxins in the water. Because of this, it should take about 15 to 20 minutes for water to pass through the media. For example, if the flow rate through the filter is 500 gallons per hour then the filter should be at least 125 gallons. In other words, at 500 gph it takes 15 minutes or 1/4 hour to fill or replace 125 gallons in the filter. I know you are thinking that means you need a very large filter. 

A final thought on what I believe to be an untried filter design. If you size the return line from the filter large enough, I believe that if your filter has removed enough of the debris you could stuff your return line with brushes. This would greatly increase the surface area inside the pipe and would probably work very well. I am thinking about incorporating this design into to my filtration system. Just make sure that you have some way to remove the brushes and that they don’t create too much restriction on the drainage of the water. If that is not taken into consideration, the filter could overflow. 



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