There is another aspect to this that very few folks ask about. What is the best flow rate to accomplish filtration? This is not the same question as asking what amount of current is right for a tank and its specific inhabitants. Often these two needs are not the same.
Slower flow rates are more idea for the filter, but higher rates may be needed for the fish or to help with circulation. Too often one uses a filter for both these purposes. In smaller tanks this may work well. But as tank size increases, it become less and less effective.
The focus of filtration is not so much on flow rate as it is dwell time, that is the amount of tine the water is in contact with the media before it exits the filter. The longer it is contact with the media, the more time the bacteria inside that media have to extract things from it or the more effective it will be at trapping waste particles. But this is just one concern when it cones to good filtration and flow rates. The bacteria we want lives in a biofilm they create. This film is home to more than just the nitrifying bacteria. As the bacterial colonies build up there is more and more bio-film and it often becomes thicker. While this biofilm is how the bacteria all attach themselves to hard surface, this stuff is not crazy glue. If the water current passing over it is two strong, it can shear the biofilm, or pieces of it, off the surface to which it is attached.
Given any flow rate, the larger the volume of media through which it passes, the more diffused the strength of the flow. I will often use a hang on model which has a way to adjust the flow and then I will get a larger filter than is needed were it run at full power. Then I run it at a reduced flow rate. This gives me the advantage of more room for media without having to go to a higher flow to get it. For example, an AquaClear filter can reduce their flow rate from from 100% to about one third. So I can get a 200 gph filter, reduce the flow to about 125 gph and have a filter with more media that will be allowed to work at the most rffrctive flow rate.
If one needs more flow for their fish or for circulation, such as circulating fertilizer and co2 in a high tech planted tank, using an added powerhead or pump accomplishes this without sacrificing filtering capacity to get flow.
