Leslie is right — there are many advantages to adding a sump to your seahorse setup. For starters, it increases the overall water volume of your system with all the benefits that implies. A good-sized sump can easily double your carrying capacity, increasing your safety margin accordingly. It makes an ideal place to put a protein skimmer, heater(s), air stones, and other equipment so they don’t have to be hidden in the display tank. (A well-designed sump does a great job of trapping and eliminating the microbubbles emitted from skimmers and preventing them from entering the aquarium, and provides an excellent way of increasing the aeration/oxygenation, which is so important for a seahorse setup.) It’s the perfect place to perform additional mechanical and chemical filtration, tailoring the filter media to meet ones exact needs, or to add a calcium or nitrate reactor or even a Deep Live Sand Bed (DLSB) to your seahorse setup. Because the sump is a large body of water separated from the aquarium itself, it facilitates water changes, dosing supplements, adding top-off water to the tank and other maintenance tasks, all of which can be carried out in the sump without disturbing the main tank or stressing its inhabitants. Entire sections of the mechanical filtration can be cleaned at one time without affecting your primary biofilter, and water changes can be performed gradually without causing stress to the fish or invertebrates. A sump/refugium can also be used to grow a lush bed of macroalgae using a reverse lighting cycle to stabilize the pH and absorb wastes.
To take advantage of these benefits, I suggest adding a two-chambered sump to your tank. This can be accomplished by installing a perforated tank divider across the width of the sump, thereby separating it into two isolated compartments. One side accommodates all of your equipment (in-sump skimmer, return pump, heaters, titanium grounding probe, UV sterilizer, etc.) while the other side can be used to establish a deep live sand bed (DLSB) with plenty of Caulerpa. The DLSB/Caulerpa side serves as a refugium and will soon become populated with countless critters (copepods, Gammarus and other amphipods, larval crustaceans, etc.). With the Caulerpa acting as an algal filter and the anaerobic layers of DLSB providing denitrification, the aquarist never need be concerned about nitrates or nuisance algae with this type of sump/refugium.
In addition, the biological refugium/sump can be maintained on an opposite light cycle to the main tank to offset the daily fluctuations in pH, photosynthesis, dissolved oxygen/carbon dioxide, and redox levels that otherwise occur in the aquarium. Daily variances in chemical, physical and biological phenomena are a fact of life in aquaria, linked to the light and dark cycles and the diurnal rhythms of captive aquatic systems. As one example, the pH of aquarium water typically peaks after the lights have been on all day at a maximum of perhaps 8.4, only to drop to low of below 8.0 overnight. This is related to photosynthesis and the fact that zooanthellae and green plants consume CO2 and produce O2 when there is adequate light, but in essence reverse that process in the dark, consuming O2 and giving off CO2. Redox levels, available calcium and other water quality parameters are affected in similar ways. Needless to say, these variations are far greater is a small, closed-system aquarium than they are in the ocean, so it’s beneficial to minimize such fluctuations by reversing the photoperiod in the main display and the sump/refugium. This is easily accomplished by timing the lighting in the sump so that the bed of macroalgae is illuminated after dark when the lights on the display tank are off, and vice versa. Just use alternating timers on the main tank and the refugium tank so that when one is on, the other is off. Voila! Just like that the roller coaster ride is over: no more daily fluctuations in pH or highs and lows in calcium levels, oxygen minima, or peaks and valleys in redox potential.
Because it is separate from the main system yet shares the same water, the sump/refugium can also be used as a nifty acclimation tank for new arrivals or a handy isolation tank for separating incompatible specimens. For seahorse keepers, the refugium compartment of a divided sump or dual chamber sump makes an ideal grow-out tank for juvenile seahorses that have outgrown their nurseries but are still too small to be kept in the main tank. A dual-chamber sump is a very versatile design that lends itself to multiple purposes. Use your imagination!
Best of luck with your new seahorse setup, Jen!