Choose a right filter type based on your aquarium size, fish types, requirements, and budget. For normal use on small – medium fish tanks (under 50 gallons), using an external or internal power filter is enough and economic.
I can't see putting an eheim on a tank that small.
External filters usually provide a good water cleaning and chemical values control. Each brand has a different design of the internal structure of the filter, anyway usually both classical and hang on filters are divided into small removable baskets or sections. Surely external filters are easier to clean because during this operation is impossible to dirty the aquarium. Moreover often classical external filters have the possibility to close and leave attached at the aquarium the way in and way out pipes, so you can clean the filter in another room, such garden or bathroom, without pouring out dirty water around.
Best filters for 3/4/5 gallon aquariums and small Betta fish tanks
The hang on external filter is a quite new filtering system. It consists of a rectangular box that hangs on the aquarium glass by a curved side. The pump is situated in the way in and gets water with a solid pipe, while the way out water creates small falls passing on the curved hang on side of the filter. These filters are really nice to see and are thought for small aquariums, anyway due the hang on part they need to be placed on open aquariums, or you have to modify the closure of the aquarium top.
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