a suction strainer the other end 6 b of the pipe 6 opens in the region of the centering preform 1 h the end 6 b of the pipe 6 protrudes from the centering preform 1 h to be connected in a watertight way by any known and appropriate means to the pumping system housed in the technical box more specifically the intake pipe ta is connected to an intake connection exhibited by the pump 7 housed in the box 2 this pump is advantageously a two speed pump fig3 and 5 show the delivery pipe tr connected to a corresponding connection of the pump 7 is coupled in a watertight way to a pipe 8 pre installed inside the reservoir 1 a and opening via a nozzle 8 a particularly in the region of the one of the risers of the steps turning once again to fig7 upper part of the support 4 containing the filter bag 3 is connected in a watertight way to a mounting plate 9 itself secured in a watertight way to the housing 1 c which is very much in the form of a chimney as shown in fig1 the entire filtration system comprising filter bag 3 and support 4 is therefore housed inside the reservoir 1 a while communicating with the water of the pool which enters the said reservoir via an opening 1 i equipped for example with a skimmer.