Next to water, aquarium filtration is by far the most important item in keeping fish alive. There are basically two types of filtration - mechanical and biological. The most common misconception is that you need a filter that has a massive flow rate to keep the water clear. Water clarity is achieved by biological filtration not mechanical, true high flow rates will keep things stirred up and eventually remove all the suspended particles from the water this type of filter is a must if you keep large fish with messy eating habits but even some large fish don't do well with strong currents so the filters water return must be defused to do away with the strong current but still retain water movement this can simply done by returning the water into the tank through a spray bar, spray bars or sometimes called rain bars are a pipe with a series of holes drilled the length of the pipe, the water returning through these smaller holes over a much greater area reduces the current but still has good water movement.
Water clarity as stated earlier is achieved by biological filtration, all filters including high flow filters develop cultures of beneficial bacteria but water needs to be in contact with this bacteria for a certain amount of time for it to do it's job, there are two types of bacteria develop in the filter the first converts ammonia the second converts nitrite (read our article the nitrogen cycle). I have had many people complain about cloudy water, cloudy water is usually caused by a bacterial bloom these blooms are common in newly set up tanks, tanks with insufficient biological filtration or where the bacteria in the filter has been destroyed by incorrect cleaning (read tank maintenance).
Retaining water in the filter is far more important than having high flow rate ideally the water should be in contact with the beneficial bacteria for 4 to 6 minutes this can simply worked out by dividing the filter water capacity by the filters liters per minute (liters per hour divided by 60), for example if the filter container has a capacity of ten liters and a flow rate of 1000 liters per hour (16.7 per minute) the time in contact is 1.7 minutes there is another equation that comes into play here that is the number of times the filter turns over the tank water for example if the tank is 200 liters and the filter is 1000 liters per hour (16.7 liters per minute) this means the tank water is turned over 5 times so now the equation is time in contact of 1.7 minutes times tank turn over of 5 so theoretical time in contact 8.5 but taking into account that only a percentage of filtered water passes through the filter again every hour this figure can by reduced by 40 to 50% therefore making this filter ideal for a 200 liter aquarium.
There are many types of filtration available and literally dozens of brands each claiming to be the best there are a few things to take into consideration when purchasing a filter.
1) Filtering surface area: If the filters surface area is to small it will block up very quickly and need servicing on a very frequent basis.
2) Filter media: Most filters come supplied with media that is either too course or too fine, course media is good in as far as it won't block up quickly but very fine particles will pass through until the media clogs, to fine a media will block up very quickly and need cleaning regularly, the best option is to use a filter that has multi stage media a course to trap large particles and a fine to trap small particles.
3) Flow rate: Rather than using one high volume filter use two to achieve the required water turnover this will give you a much bigger filtering surface area and better biological filtration.
4) Price v reliability: The old saying “You only get what you pay for” certainly is true when it comes to aquarium filters, many people have lost very expensive fish because they wanted to save a few dollars and purchased cheap filters, we have tested some of these cheap filters and the results are far from satisfactory some totally failed within 3 months and most needed impeller replacement with in 6 months.
No matter how good your filter is it will not do away with the need to do regular water changes and tank maintenance or allow you to over stock your aquarium.