My rationale for incorporating an algal turf scrubber (ATS), into a reef aquarium is two-fold. Firstly, I wanted to provide a habitat for micro-crustaceans, worms and the like to live (see Raabe 2008).

These organisms are useful because they help to process organic waste and help clear the water of dissolved organic compounds. They also produce planktonic eggs and larvae that drift into the water stream, providing live food for filter feeding organisms in the system as a whole.
Secondly, an algal scrubber would provide a means for nutrient export through the harvesting of fast-growing turf algae (Raabe 2008).


Design and implementation
Turf algae grows best in the areas of a reef tank where there is a lots of oxygen and a high level of gaseous exchange. The ATS described here is nothing more than a simple perspex or glass screen over which water is allowed to flow. The screen is placed within a portion of the sump and provides a suitable environment for the turf algae to grow.

When designing my sump I had in mind that I would be incorporating an ATS from the start, but this can also be achieved within an existing sump arrangement by diverting some of the overflow over a screen or plate before entering the sump. Another way to provide a good habitat is to lower the water level in the last compartment of a sump so that the water flows down a vertical glass pane.

I had the sump designed as two sections, primarily because it needed to replace an existing arrangement and this was the only way that I could get a large sump into my cabinet. As seen in the diagram here, the first section of the sump has one end lower than the rest. This allows the water to overflow from this end, across the glass pane of the ATS, and into the second section of the sump.

Certain features of the sump system, such as the baffles that are essential to it’s correct operation, have been left off this diagram for the sake of clarity
The picture below shows the second section, with the overflow part of the first section shown on the right. As you can see, fixed to the first baffle in the middle is a thick piece of glass. The ATS pane is designed to be removable, it rests on the top brace to the right, and this thick piece of glass to the left. Water is designed to flow over it and into the compartment underneath.

The next picture shows the another view of the second section, this time installed underneath my tank. Note that I have not yet put the screen that makes up the ATS in place yet; at his point the water is simply flowing from the first section of the sump (mostly out of view) into the second section of sump visible here.

The ATS screen
In essence, an ATS is nothing more than a flat sheet across which algae can grow. The next photograph shows the perspex (acrylic) sheet initially used for this. The transverse bars across were for strengthening. I have since replaced the perspex sheet with a glass pane, as I found that even with the strengthening bars in place the perspex sheet would bow underneath a heavy growth of algae.

The screen is placed on the right hand brace and the supporting bar on the baffle to the left. Water flows over the sheet from the first stage of the sump and into the second stage of the sump underneath.

Seeding the algal scrubber
Some turf algae from my reef tank is placed on the screen and temporarily held in place by some flexible tubing to ‘seed’ the ATS. It takes a week or so for the turf algae to get a decent hold, after which the tubing can be safely removed. As you can see in the photograph, diatoms are starting to build up on the ATS after only a couple of days.

The next photo shows the scrubber two months later, just before I cropped it for the second time. As you can see, the turf algae has nearly completely covered the screen! Lighting over the ATS is two 15w normal-output marine white fluorescent tubes. There are two compact fluorescent marine white bulbs (36W) lighting the other side of the section over the Chaetomorpha; some of this light is presumably being utilised by the turf algae.

It would be interesting to analyse the algae and detritus on the algal detritus scrubber with a microscope to see what types of life there are there. Since I do not have such equipment (yet!) I have had to make do with a magnifying glass. So far I have found copepods, amphipods, isopods and various worms, mostly of the bristleworm (polychaete) variety. I am glad that I married my wife before she caught me looking at algae through a magnifying glass; her face when I was ‘caught in the act’ spoke volumes!


Results
Alongside fitting an algal turf scrubber, my sump/refugium arrangement has also been improved through the addition of a remote deep sand bed (RDSB), the addition of a live rock section, and an expansion of the water volume. Because of these changes, I can’t say precisely how much of a difference the incorporation of the ATS has made by itself. However, taken as a whole, these changes have benefitted my reef tank, notably with increased growth rates among the scleractinian corals and generally excellent health among the inhabitants, including the fishes.

Nitrates in the system have been reduced from about 5 ppm average to just a readable trace since these changes. Phosphates are easier to control and keep at the lower end of the scale, between 0.001 and 0.003 ppm. Chaetomorpha were already being grown, so the combination of increased water volume, RDSB, and ATS must be responsible for this. The aim was never to reduce these parameters to zero, as both are necessary nutrients. But both nitrate and phosphate do, however, occasionally show up zero; when this happens then the feeding is increased. In fact I am now able to feed the system more frequently, and have added an automatic feeder to provide dried (pelletized) food throughout the day. It would be interesting to see how a similar system would react if only the ATS was installed, and none of the other changes mentioned above.

When the turf algae is cropped or purposely disturbed, sending the cloud of brown detritus collected by the algae (with whatever else is in there) into the water stream, coral polyp extension is seen to be exaggerated on certain corals. In particular I have noticed Hydnophora, Goniopora, Merulina and Galaxia spp showing this reaction.

The turf algae is cropped once every 2-3 weeks, and I take approx. 75 grams each time (water squeezed out before weighing), leaving about half the algae on the screen to grow back. To crop the algae I just gently pull it away from the screen taking care not to pull the whole lot off at once. It comes away quite easily as the glass is obviously smooth. This is not a problem, but a potential improvement would be to have a textured surface on the glass pane to make it easier for the algae to take hold.

This amount is compared to about 250 grams of Chaetomorpha that I take at the same time. So, using a basic measurement method, assuming that both types of algae sequester the same amount of excess nutrients, the ATS increases the nutrient export of the algal side of my system by about 30%.

As we all know, space is at a premium in most reef tanks, and inside the cabinet itself is no exception. In my opinion, an algal turf scrubber makes a useful addition to a fishless refugium design, taking up very little space while delivering benefits in terms of water quality improvement and the production of planktonic food.


Acknowledgements:
I would like to thank Charles Raabe for his inspiration on this project, and also Neale Monks for his input with the content and help with the diagram.


Reference
Raabe, C. A Philippine Fringing Reef & The Reef Aquarium
Chucksaddiction.com (2008)