It is obvious that the external filter is placed outside of the aquarium, so you need to plan in advance for it. Users claim that in the operation of the external filters is much more convenient than internal, as they are always available for service. Before you buy first and foremost is to find out what capacity must possess your future filter. It depends on the size of the aquarium.
If you are the owner of a small aquarium, mainly pay attention to the quality and noise performance, because the power requirements of the filter is small. Carefully study the recommendations of the manufacturer, the volume of your tank should be slightly less than the recommended norms, otherwise the filter may not cope with their work. If your aquarium is quite large, consider also its densely populated. It happens that the inhabitants very much, in this case you should buy a filter with a large power reserve.
For small aquariums with a volume of from 30 to 70 liters external filter must have a bandwidth of from 300 to 400 liters per hour. That should be enough, even if artificial pond inhabited densely enough. In this case, you can buy such models of external filters, such as Filter Aquael MINI KANI 80, Aquarium Systems MILLENNIUM 1000 and Tetratec EX 400.
If we consider another group of aquariums, the volume of which is from 60 to 100 liters, filter capacity should be more. The average for this group it is 500 liters per hour. As for specific models - it is possible to pay attention to the external filter Tetratec EX 600, the manufacturer's recommended amount of aquarium up to 120 liters.
For larger aquariums range of external filters is very diverse. Throughput, naturally should be higher. For example, for a volume of 100 to 200 litres on average it is 700 l/h. This model has a power filter EHEIM Professionel 2224, it is designed for the aquarium up to 250 liters. To date, the public external filters, designed for huge volumes. For example, the model EHEIM Classic 2260 is able to maintain the aquarium volume up to 1500 litres, with annual capacity of 1900 liters per hour. The range of external filters is really wide, so it will not be difficult to find the right model for you.