With the right kind of hang-on-the-back filters, you can successfully run a saltwater aquarium, but sumps do have their advantages. Sumps increase the total water volume of your system, which helps stabilize fluctuations in temperature and chemistry.
Sumps give more room for equipment like protein skimmers, phosphate reactors, and biological filtration while keeping them out of view of the display tank. Also, when the water flows down into the sump, it becomes more aerated. Sumps make it easy to add a freshwater auto top off system so the water level doesn’t go down as evaporation happens.
Finally, when adding any dosing supplements, it gives more time for them to mix in the water before they go into the display tank. If your tank seems to be in a slump, ask an Into The Blue team member about building a custom sump for your aquarium!