Below tank refugiums usually receive raw feed water from the display. This water is rich in organics from surface tension at the air water interface as well as heavy in stirred up detritus or excess food that gets swept into the overflow. Hence, this design is great for nutrient uptake and removal. Fast growing macroalgae like Chaetomorpha or Caulerpa can utilize these nutrients and convert them into living tissue for easy removal from the system. You will also notice that in dense growths of algae a variety of critters are attracted to this ever-flowing food source. Mini brittle stars, amphipods, copepods, and mysis shrimp are some of the most common ones you will find in amongst the macroalgae growths. Again, these living creatures are there to grab a hold of any available food and convert that into living tissue. While they are unable to use every portion of that food to create tissue and do create some waste products, they are living amongst the macroalgae. In effect, the algae is capable of consuming some of their waste products just the same as the overflowing waste from the display in a symbiotic-like relationship. Additionally, below tank refugiums with their normal multitude of baffles to trap and eliminate bubbles also tend to trap detritus. In effect, these baffles tend to operate as a settling chamber and when the hobbyist routinely siphons out this debris, they further facilitate nutrient export. As you can see, these below display designs operate optimally for nutrient export. And while they do harbor a number and diversity of planktonic lifeforms, to get up into the display they must pass through the return pump. This surely impacts the number of viable specimens that make their way into the display even though the exact amount of impact is debatable. Sperm, eggs, and small larvae could conceivably pass through unharmed, but larger adults are likely to become shredded. Although pulverized, dead crustaceans are still edible just as live, whole ones are, if the intent is a steady supply of live plankton to feed into the display, there are better placement options, which leads me to discuss the next installation option.
Above Tank
This design is by far the best for introducing live plankton into the display because they are gently gravity fed into the aquarium from the refugium versus traveling through the pump to make their way into the display. This ensures that both offspring, reproductive material (eggs and sperm), and adults alike can re-colonize the heavily grazed and preyed upon display. For as good as this layout is for plankton production, it unfortunately is not nearly as good for nutrient uptake and removal as the below tank design assuming that the system utilizes both a sump and above tank refugium. In this scenario, the water coming to the refugium from the sump is likely to already be heavily scrubbed of nutrients by the sump’s protein skimmer, by chemical filtration media located there, mechanical filtration, and/or possibly another refugium.
The other drawback to this layout is it is not as easily incorporated into most displays. It can be retrofitted fairly painlessly into tanks that are built into a wall. In these instances, a portion of the return pumps water can be diverted and sent to a secondary aquarium mounted on a secure shelf above the display. Unfortunately, not all of us have setups such as this. Most hobbyists are working with commercially available aquarium and stand kits. This makes creating a seamless appearance of an above tank refugium with a display tank difficult.
Best of Both Worlds
It should not be a great leap in logic to see that combining two refugiums, one above tank and one below tank, will theoretically yield the best results. It just takes some planning and forethought to incorporate both and is not easily retrofitted into existing displays unless you have a considerable amount of room to work with, as is the case with in the wall designs. But, two refugiums also gives you additional options as far as differing the substrates in each or varying the lighting schedules (all subjects that will be discussed in future editions). For instance, the below tank refugium could focus on macroalgae growth while the above tank refugium’s primary goal was generating amphipods, copepods, and mysis shrimp. Or, the below tank refugium could grow macroalgae with a 24/7 lighting schedule while the above tank one could operate without lighting to create a cryptic zone that promotes feather dusters and/or sponges for their filter feeding capacity. Truly, the options are limitless. One only needs to research a little to custom tailor a refugium package that best suits their specific aquatic pets’ needs.