The larger models pose a considerably greater challenge in this regard due to their increased bulk. They are thus more likely to find a place in working rather than decorative aquariums. A variety of companies market very similar power-driven modular sponge filters. Sometimes identified as submersible or internal power filters, these units house a hollow cylindrical sponge mass inside a flow-through plastic housing. A top-mounted pump pulls water through the sponge, whose central cavity can be filled with chemically active media.
Return flow is regulated by a preset internal control.
Efficient, externally mounted suction cups hold these reasonably unobtrusive corner units firmly in place against the tank wall. These filters are absolutely silent and quite economical in their power consumption. The total working area of their sponge masses is much smaller than that of air-driven units. Their waste-handling capacity is thus proportionally smaller, which makes them a poor choice for heavily stocked aquariums. The sealed-bearing motors of these filters use the tank's water as a coolant.