Superior filtration technology with built-in functions The most recent iteration of our professional external filters is called Professional 4+. Sturdiness, a large filter volume, corner compatibility, and compact size are all features of the square canister. The new Xtender function allows for longer intervals between filter cleanings. There are three sizes of Professional 4+ filters for aquariums up to 250, 350, and 600 liters. Thermo filters (T) are another option for the 250 and 350 models. The high-tech Professional 4e+ 350 is the most sophisticated model; it can be controlled electronically from a PC (see Professional 3e). High flow capacity, low energy consumption, and other characteristics define all Professional 4+ filters: No self-priming