The Refugium

By Refugium we mean nothing more than a small aquarium connected to the main tank, dedicated to the breeding and reproduction of small and medium-sized organisms and to the cultivation of macroalgae. In a separate environment far from predators and phytophagous fish, large quantities of small organisms deriving from living rocks and sand or specially introduced: adult or larval individuals of Protozoa, Crustaceans (Crabs, Shrimps, Copepods, Mysis, Branchiopods, Ostracods, Amphipods, Hermit crabs), Gastropods, Echinoderms (Ophiures, stars marine), Worms (Oligochaetes, Polychaetes, Nematodes), etc.


It is also possible to introduce small animals (such as a couple of Lysmata shrimps) which usually expel eggs or larvae which in a traditional aquarium would be immediately eliminated by the pumps, filter or skimmer.

Other organisms that can increase the population are hermit crabs, Turbo snails, starfish, spirographs, and a large amount of Macroalgae (Caulerpa, Halimeda, Rodophytae, etc).

What is the Refugium for
The refugium is an excellent complement to the Berlin method, a great help to the autonomous ecosystem that one wants to obtain in the reef aquarium. The advantages are many and important:

  • 1) Increase the total volume of the aquarium
  • 2) Possibility of breeding spontaneous live plankton as food for fish and invertebrates.
  • 3) Breeding of detritivorous animals, the Refugium becomes a sort of mechanical filter, where the detritus is consumed by microphagous organisms.
  • 4) Possibility of introducing small animals that produce eggs and larvae (which are not immediately eliminated by the filtering system).
  • 5) Hatchery for Artemia (the use of shelled eggs is recommended)
  • 6) Cultivation of Macroalgae which helps to keep polluting substances under control (Nitrates, Ammonium, Phosphates) and through photosynthesis they consume CO2 and release oxygen to the water (this helps to keep the pH stable). The grown Macroalgae can be pruned and used as food for phytophagous fish.
  • 7) The substrate of the refugium which must be made up of a high layer of coral sand to favour the colonization of detritivorous organisms, over time is colonized by anaerobic bacteria which reduce the nitrate content.
  • 8) Using the refugium with an inverted photoperiod, thanks to the CO2 assimilated by the algae during the night, the lowering of the nocturnal pH is contrasted. In this case, in order not to disturb the aquarium animals during the night, it is advisable to equip the refugium with an opal glass lid, apply a removable flap (in wood or PVC) to the front glass and darken the remaining three glasses with black adhesive paper.
  • 9) By introducing the water enriched with CO2 from the calcium reactor, the growth of the Macroalgae bred in the refugium is promoted and the content of free CO2 in the aquarium water is reduced.
  • 10) Lastly, observing the behaviour of the organisms bred in the refugium is just as interesting as observing the animals in the aquarium.

How does it work
As we have said, the refugium consists of a small tank positioned next to or above the aquarium.

One problem is that the impeller of the pump used to feed it can kill some of the plankton it comes into contact with.

For this reason, it is good to take the water from the aquarium with a small pump, put it in the refugium and let it go back into the aquarium due to overflow through a hole made in the upper part of the refugium.

In this way, no organism will ever come into contact with the impeller of the pump. The refugium must be illuminated, I recommend installing at least a couple of timer neons. The photoperiod can be extended up to 14 hours.

For the realization of the refugium, all types of artisanal or branded aquariums can be used, even a used aquarium can be fine or even a tub or a plastic container.

In the case of new aquariums, it is possible to integrate the refugium directly into the main tank (next to or behind), this gives the structure a better aesthetic impact.

A) Aquarium
B) Refugium
C) Pump that pushes the water into the refugium
D) Overflow (exit by gravity from the refugium).
E) Compartment to contain the neon lights and the timer starter.
F) Bottom 6-8 cm coral sand (two layers, medium/coarse below fine).
G) Live rocks and macroalgae

The water flow must not be too turbulent but of medium-low intensity (around 2-4 total changes per hour).

In the case of very small organisms, such as Artemia nauplii for example, there is a real risk that they will be introduced into the turbulent water of the aquarium a few hours after hatching with the consequent immediate elimination by the filters, movement pumps, and from the skimmer.

To minimize this inconvenience, there is the possibility of feeding the refugium with two pumps managed by timers. A pump that works continuously must have an extremely low flow rate, sufficient to guarantee good conditions, but which limits the escape of microorganisms from the refugium (in this way they can grow and reproduce undisturbed).

A second more powerful pump introduces a greater quantity of water occasionally and for a few minutes, possibly with the simultaneous switching off of all the movement pumps and the skimmer (for about an hour).

A nylon mesh pre-filter (those used in the mechanical pre-filters of osmosis systems) could be inserted before the refugium overflow (positioned horizontally above the water level) to completely eliminate the risk of plankton leakage during the period managed by the pump small.

The pre-filter must have a longitudinal slot in the upper part to ensure free flow when the more powerful pump starts (with the consequent slight rise in the water level in the refugium), and in the event of a blockage of the pre-filter it would prevent the overflow of water from the refugium.

I leave to those who want to experiment with this solution any tricks and modifications, especially on the interval times of the two pumps, and on the size of the lattice to be used (in fact, I fear that a lattice so fine as to prevent the Artemia nauplii from passing will clog with surprising speed).

The furnishings of the refugium are very spartan. 6-8 cm of coral sand is sufficient (half below medium/coarse, and half above fine) and a few kg. of live rocks (possibly not all from the same place).

Excellent results have been obtained with the use of live sand, unfortunately almost unobtainable in Italy. Some sand from a mature aquarium might be a good alternative.

Subsequently, various types of Macroalgae will be introduced.

The refugium should be fed as if it were a small aquarium. All types of feed that we use for our fish can be used such as flakes, frozen, and granulated (clearly in small quantities).

The liquid products used to feed invertebrates and products based on Zooplankton and Phytoplankton are also excellent (during administration it is advisable to switch off the feed pump for a short time).

Macroalgae find nourishment directly from the water and from the trace elements that we administer to our invertebrates.<

Sump and refugium
The solution of inserting the refugium in the sump would seem the most convenient and immediate, unfortunately, the impossibility of not letting the water pass through the pump limits its functioning, as already mentioned, the impeller would partially kill the plankton.

Even if many organisms would be able to pass unscathed, and in any case, even if they were killed they would probably still be preyed upon, the solution is less efficient than that of the refugium positioned above or beside the aquarium.
The advantages given instead by the cultivation of Macroalgae would remain unchanged.

Although not recommended, below is the scheme for the construction of a sump equipped with a refugium. To make the most of the length of the sump, the tank for automatic topping up has been designed narrow and long in the rear area.

In the case of a refugium inserted in the sump, the use of Tunze pumps is recommended, in fact, the latter has a much larger compartment in which the impeller is contained than that of the other pumps. Tunze pumps are the only ones that partially safeguard the plankton.

The hypothetical scheme is structured for an aquarium 150/160 long x 60 deep. In this case, the refugium is very large if we assume the measurements of about 60/70×35/40×60(h) we have a gross volume of 120/170 litres.

Side view:
A) Water from the aquarium
B) Mud collection compartment
C) Skimmer pump
D) Skimmer
E) Mechanical filter (sheet, pocket, etc.)
F) Activated carbon compartment
G) Delivery pump
H) Water in the aquarium
I) Tap to regulate the water in the refugium
L) Compartment to contain the neon lights and the timer starter.
M) Refugium
N) Overflow (exit by gravity from the refugium).
O) Water entering the refugium
P) Live rocks and macroalgae
Q) Bottom 6-8 cm coral sand (two layers, medium/coarse below fine).
Top view:
A) Automatic top-up water storage tank
B) Mud collection compartment
C) Skimmer
D) Refugium
E) Sump
Elizabeth Canales is fond of marine life since childhood and got an aquarium as a gift from her father on her 6th birthday. Since then she started to take care of Coralia (her goldfish). Her love for fish made receive her B.S. in Marine Biology from the University of Washington and DVM from Delaware State University.

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