Over filtration in the case of the two animals mentioned will not benefit you as much as proper maintainance will. However the Marineland 250 is by no means over filtering a 40 gallon. The package may say the filter is rated for a certain size, but the ratings are baked. 250 gallons per hour pre-media is not too much for a 40 gallon. Your idea of double filtration and mine are off a bit.