A sump works equally well in a reef,or a fish only tank.Not only can you equip them to filter your water,,,but they make a great place to hide all the things you dont want too see in your beutifull tank.Like the heater,,,skimmer,,,and thermometer.They also make water changes easier.And topping off for evaporation is also done here so you done disturb the tanks occupants with fresh water.