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2.9 Algae Scrubbers (long)


This article is from the ReefKeepers FAQ, by several authors (see the Credits section).

2.9 Algae Scrubbers (long)

Summary: the jury is still out. May help, may hurt, not currently
recommended, especially as the sole filter. The topic is
controversial. Below is some discussion about it.

In most healthy natural communities, particularly coral reefs,
dissolved nutrients are scarce. In aquaria, by contrast,
nutrients in the form of dissolved inorganic nitrogen, or DIN, (a
collective term for ammonia, nitrites, and nitrates) accumulate
very rapidly as fish and other organisms excrete these wastes.
The most basic problem in any aquarium is limiting the
accumulation of DIN.

In reef aquaria, DIN is consumed by the community of organisms on
the live rock. It is uncertain what relative contribution is made
by bacteria as opposed to algaes, but it is certain that the live
rock community as a whole can remove a substantial amount of DIN
from a reef aquarium. In fact, it is quite possible to run a reef
tank with no biological filtration (DIN consumption) other than
that which takes place on the rock. This method is part of what
is now known in the United States as the "Berlin school" of

Other schools of thought utilize additional biological filtration
in separate filters. Traditional reef tanks supplement the
filtration provided by the reef (often not acknowledging the role
of the reef itself) with bacteria-based trickle filters. Many
readers probably learned this technique first, as it has been the
dominant method in the United States amateur hobby for some time.
Yet another approach uses algaes, which are also capable of
utilizing inorganic nitrogen directly. An algae filter, or algal
scrubber as it is usually called, is simply a biological filter
which utilizes a colony of algae rather than bacteria as consumers
of inorganic nitrogen.

Algal scrubbers are not new; they are discussed in Martin Moe's
(1989) excellent _Marine Aquarium Reference: Systems and
Invertebrates_, for example. However, algae filters have been
regarded in the past as too bulky and inefficient to be the sole
filter for a aquarium. The recent surge of interest in algal
scrubbers seems to have been generated by Adey and Loveland's book
_Dynamic Aquaria_ (1991). They discuss both techniques which
allow an algal scrubber to be compact and efficient and also a
number of arguments as to why they are preferable to other
filtration methods.

One reason to use an algal scrubber according to Adey and Loveland
is that it mirrors the way DIN is cycled in nature. They claim
that perhaps 70-90% of the DIN in reef communities is consumed by
algae, rather than by bacteria. The two methods produce rather
different water chemistry; for example, algae are net producers of
oxygen and remove carbon dioxide, while a bacterial filter
consumes oxygen and produces carbon dioxide. They argue that it
should be easier to maintain the type of water chemistry found
over a natural reef by relying on an algal scrubber.

Also, algae remove the nitrogen from the water in order to build
tissue, while filter bacteria simply put it into a less toxic
form. The excess nitrogen can be removed completely by periodic
algae harvests, while dissolved nitrogen in the form of nitrate is
not as easy to remove. Adey and Loveland claim that their methods
can bring levels of DIN down to a few hundredths of a ppm, far
below (in their opinion) the levels reachable with other methods.
A related argument in favor of algal scrubbers is that stability
in natural ecosystems seems to come from locking up nutrients in
biomass, not in allowing it to be free in the environment. An
algal scrubber does precisely this, while a bacterial filter
converts it to free nitrate dissolved in the water.

A final reason to use an algal scrubber according to Adey and
Loveland is that many other kinds of filtration (including protein
skimmers) remove plankton from the water. An algal filter
naturally does not do this, and can actually provide a refuge for
some forms of plankton. The importance of this effect is,
however, a matter of some debate.

As compelling as some find the above arguments in theory, there
seem to be serious problems with algal scrubbing in practice.
Many attempts by public aquaria at implementing reef tanks using
only algal scrubbing have been failures. In particular, it seems
difficult to find successful long term success with Scleractinia
(stony corals) in such tanks, and those success stories which can
be found are quite difficult to verify and often contradicted by

Various public and private aquaria have used algae scrubber
filters on their reef aquaria, with disastrous results. The
microcosm at the Smithsonain Institution has yet to keep
scleractinia alive for more than a year. While Dr. Adey has stated
how well corals grow in this system, those viewing the system have
failed to find these corals. In an interview with Jill Johnson,
one of the techs responsible for the Smithsonian tank, she stated
to Frank M. Greco that frequent collecting trips were needed to
keep the system stocked with live scleractinia.

The Pittsburgh AquaZoo also has a "reef" tank based on Dr. Adey's
algal scrubbers. This tank is nothing more than a pile of rocks
covered with filimentous green algae, and the water is QUITE
yellow (as is the Smithsonian tank) from the presence of dissolved
organics (ORP readings have been around 165). As with the
Smithsonian tank, scleractinia do not survive longer than a few
months. The same applies to soft corals as well. When I (Frank M.
Greco) saw this tank on May 3, 1993, there were NO living corals
to be found even though a collecting trip to Belize was made
several months earlier and 81 pieces of living scleractinia were
brought back. There were, however, two piles of dead Atlantic
scleractinia: one right behind the tank and the other in the
greenhouse housing the algal scrubbers.

The Carnegie Science Museum (Pittsburgh, PA) also uses an algal
scrubber system, but with significant modifications. This tank
looks the best of the three. There are several species of hardy
Scleractinia and soft corals that are doing quite well. The water
is clear (a bit cloudy). The major differences between this system
and the other two is the use of carbon, a small, barely
functioning algal scrubber, about 1000 lbs. of excellent quality
live rock (Florida), water changes, and the addition of Sr and Ca.

The last system I know of that uses an algal scrubber is the Great
Barrier Reef Microcosm in Townsville, Australia. As of this
writing, the system is not maintaining live Scleractinia, and
frequent collecting trips are needed in order to replenish the
exhibit. It should also be noted here that while Dr. Adey has
claimed in his book Dynamic Aquaria that corals have spawned in
this system, what he doesn't mention is that the corals which
spawned were collected only months before the known spawning
season. From these few examples, it should be clear that algal
scrubbers are NOT to be used in systems containing live

Possible reasons why algal scrubbers seem to fall short center
around the observation that it seems difficult to control hair
algae growth in scrubbed aquaria. Hobbyists have for many years
seen their stony corals slowly pushed back off of their skeleton
and killed by encroaching algaes, and much effort in the hobby has
been devoted to controlling this growth. Only with strict control
of algaes does coral survival seem possible. Most or all reefs
with algal scrubbers seem to have heavy algal growth in the tank
as well, which the experience of the hobby suggests is
incompatible with stony coral survival.

The main method used by hobbyists to restrict algal growth is to
reduce nutrient availability; in fact, the claim that other
methods cannot reach the same low levels of DIN achieved by algal
scrubbing is probably not true. Advanced hobbyists are beginning
to use better tests, such as HACH's low level nitrate test, and
are finding that they can achieve nitrate levels below 0.02 ppm.
Berlin methods seem particularly able to reach these levels, which
are comparable to that on natural coral reefs.

If low nutrient levels can be achieved by both methods, then why
is algal growth a much greater problem with scrubber methods? The
answer is not known, but there are two factors which probably

First, the discussion so far has mentioned only inorganic
nitrogen. Algaes seem to release much of the inorganic nitrogen
which they take up in the form of dissolved organic compounds
(DON), which can also be later utilized by algaes. The very low
levels of DIN measured in scrubbed tanks may mask the very high
levels of DON which persist, providing nutrients for strong algal
growth. This is borne out by many reports that the water in
scrubbed tanks often has a pronounced yellow cast, characteristic
of dissolved organic compounds. Since the water over natural
reefs is very low in DON, high levels may be directly harmful to
many corals, in addition to promoting uncontrolled algal growth.

Another possible effect of algal scrubbing is more subtle. Algal
growth is never completely halted in any marine tank, merely
reduced to the point where macro- and micrograzers can keep them
in close check. The net rate of new growth depends not only on
the availability of nutrients, but also on the amount of existing
algal growth releasing free-floating cells into the water to
colonize new sites. Even if the rate of growth of individual
algal colonies is equal, a scrubbed tank has a growth of algae in
the scrubber much larger than a reef tank with little algal growth
anywhere in the system. This possibility suggests that the
presence of the scrubber itself and not merely high levels of DON
is an obstacle to the successful long-term maintenance of stony

The weight of evidence at this point seems to be against the use
of algal scrubbing in reef tanks, and the method should be
considered to be highly experimental. Beginners particularly are
advised to avoid this technique until they have considerably more
experience with reefkeeping. The advanced aquarist may well wish
to experiment with this interesting and controversial method, but
it would be unwise to risk the lives of an entire reef tank full
of coral. Such experiments should progress slowly, beginning with
the most hardy of inhabitants. Many of the objections center on
stony coral survival, and it is possible that scrubbed tanks with
fish and hardy invertebrates may do quite well.


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