A filtration device (10) for aquarium tanks comprises a body (11), divided using special baffles (12) into a set number of intercommunicating chambers (I, II, III etc . . . ) can be housed so that during the filtration process the water follows a zigzag route; it al comprises an infeed point (15) for the water to be purified, a temperature regulation unit (13) and a pump (14) enabling the recirculation of the water. The body (11) is fitted entirely inside the tank and the filtration elements (A; B; C etc . . . ) are located between at least one pump (14) and at least one infeed point (15) to enable entry of the water into the filtration system mainly from below upwards and exit of the water from above downwards.