The seawater extracted from a depth of 30 m has a constant temperature of about 18degreesC. This water can be used as cold source for the conditioning of Salammbo Museum aquariums (Tunisia). With this intention a passive system, using capillary polypropylene exchangers, is carried out. The tests show that this system has a good thermal efficiency of about 80%. In order to size this system, a numeric model is elaborated. The checking of this model shows a good agreement between the numerical results and measurements taken.