Algal Filtration In Reef Aquariums


Algal Filtration In Reef Aquariums
By Bryan of Santa Monica Filtration

With modern reefkeeping, more people are wanting to filter their tanks the same way that all natural reefs (and lakes and rivers) are filtered, which is by using algae.

We are going to look at the main types of algal filtration methods you can put on your system:
All grow macroalgae, which for our purposes is the same as “macro” or “algae” or “seaweed,” and all can be DIY’d or purchased. We at Santa Monica Filtration make and sell the turf scrubber types of units, but the concepts apply to all types of algal filtration.


How Algal Filtration Works

The most basic concept of natural algal filtration is this: Algae absorb “bad” things (ammonia, nitrite, nitrate, phosphate, CO2, metals) out of the water, and leave “good” things (food particles) in the water. It’s very important to understand the difference and to know that fish waste is considered food particles for corals.

You might think of cooking a steak on a grill, and this food would be a “good” thing, but the smoke that comes up from it would be a “bad” thing. Yes, the more steak you cook and eat is good, but the more smoke you get is bad. Of course you could remove the steak from the grill, and this would remove the smoke, but now you have no food. So the solution is to get a fan to remove just the smoke, but leave the food. This is how algal filtration works; it removes the smoke, but leaves the food.

The difference between scrubbers, reactors and refugiums lies in the conditions they give the algae to grow.

All oceans, reefs, lakes and rivers are naturally filtered by algal photosynthesis. This means that algae does all the filtering of these waters. This is why algae is at the base of the entire aquatic food chain, and why algae biomass dwarfs the biomass of all aquatic creatures combined 10 to 1. But for algae to absorb nutrients out of the water, the algae must grow. And to absorb nutrients faster, algae must grow faster. This means faster filtration for you.

We are commonly asked what kind of algal filtration device is “most effective.” All macroalgae operate basically the same, chemically. They all use light, photosynthetically, to absorb nutrients (ammonia, nitrite, nitrate, phosphate, CO2, metals, bad!) and to grow biomass (good!). Just like trees. Yes, you heard that right: Nutrients are “bad.” Don’t confuse them with “nutrition,” which is food.

Nutrients are bad; Nutrition is good. I didn’t make up this naming system, so please don’t yell at me when it gets confusing.


Speak Like a Geek: Basic Macroalgae Terminology
  • Chaeto: This is the nickname for Chaetomorpha macroalgae, and is pronounced KAY-toe. It looks like a green dishwasher cleaning pad and has no "roots", so it does not attach to solid surfaces and can break up and wash away. It grows in saltwater only, and is not eaten by many fish.
  • Green Hair Algae: This is nicknamed GHA macroalgae, and it has "roots" which attach to solid surfaces to keep it from flowing away. It is eaten by almost all herbivores, especially tangs.
  • Macro: This is the short name for macroalgae. Hobbyists usually mean Green Hair Algae when saying it, but technically, macroalgae is all the types of algae we cultivate, and should not be confused with “micro” algae which makes green water. Nobody cultivates micro algae purposely in the hobby. Just macro algae.
  • Slime: A solid algal growth macroalgae, bright green to brown to black in color, which attaches to solid surfaces but not very securely. It won’t be addressed here.

Which Algal Filtration Device is Most Effective?

The ways (or devices) that we cultivate and grow algae with are the big things that reefers will want to know about and consider for their systems. There are some biological differences between the ways or devices, but more important is the size, cleaning, cost, and safety of the systems; so that’s what we will cover.

The main categories are:
  • Refugiums: A traditional method of cultivating/growing algae. It usually grows GHA slowly, in a large space, and with a small light. It does not use an air/water turbulent interface.
  • Algae Scrubbers: Also called a Turf Scrubber, or Algal Turf Scrubber (ATS), or for some of the devices we make, an Upflow Algae Scrubber (UAS). It is a device that allows air and water to interact to create a turbulent air/water interface like waves on a beach; it grows green hair algae or slime that attaches to solid surfaces, and it has a light which causes the growth.
  • Chaeto Reactors: A device that has water running through it, with chaeto growing in it. Also known as an "algae reactor". A chaeto reactor does not allow air to enter; only water, and these reactors usually have a lid attached with screws to keep water in and air out. A light is placed inside or outside the reactor. (“Reactor” just means a container.)
Reactors and scrubbers are different from refugiums; a refugium (“fuge”) is a space in a sump, and of course you might already be using a sump. Scrubbers and reactors however are separate devices. A refugium is created in a sump when you place macroalgae there, and put a light on it. Refugiums have very slow flow, low light penetration, and no air/water interface; so compared to reactors or scrubbers they require the largest size to provide the same amount of filtration.

You could modify a refugium to be a reactor, and with more mods you could make it a scrubber. But then it would no longer be a refugium.

First, safety. All refugiums are DIY, so be sure the light can’t fall down into the water, or get splashed, which are the two things that happen most. Lights for scrubbers and reactors must also be kept very dry, or be submersible. The most common failure is when Chinese “waterproof” lights are used. These bolt-on lights were made for gardens and patios with occasional rain; they are not waterproof at all, and are really not even water-resistant. Worst of all, they bring the household line voltage of 120 or 240 volts right near the water. If you drop these lights in the water they will immediately short out, and salt spray can get inside them within a few months. So keep them very dry.


The Differences in Types of Filters

Biologically, the faster the algae grow, the faster they absorb nutrients (ammonia, nitrite, nitrate, phosphate, CO2, metals, bad!) out of the water. Once you remove the algae from your system, the nutrients are gone (good!) until you feed again which will cause the animals to make more nutrients. Or, if feed the algae to your fish, the nutrients stay in your system but you don’t have to add any more by feeding.

The main things you can control to cause faster algae growth is illumination, growth thickness, water flow, and air/water interface turbulence.
  • Illumination: Generally, the stronger the light, the more photosynthesis and filtering, up to a point. If too strong, you get “photoinhibition” which stops the growth. But for almost all situations of DIY and purchased filters, the light could be stronger. Scrubbers generally have the strongest lighting, followed by reactors, followed by refugiums.
  • Growth Thickness: When growth gets too thick, it blocks light and water from reaching the insides of the growth, causing the insides to die and put nutrients back into the water. It does not put good things back into the water; it puts bad nutrients because of the bacterial breakdown of the growth. Growth thickness is a primary reason why a fuge filters so slowly and thus requires the largest amount of space. At any given time, most of the “ball” of growth in a fuge is not under the light, so it is not growing or filtering. Or worse, the inside of the growth is dying in darkness. So you make up for this by using a very large size, sometimes half of an entire sump. Reactors limit the growth thickness to the size of the reactor container, and scrubbers keep the growth very thin by limiting it to an attachment surface.
  • Water Flow: Familiar to most hobbyists, good flow is needed to bring nutrients to the algae. Flow tends to lack in refugiums, but is stronger in reactors, and very strong in scrubbers.
  • Air/water Interface Turbulence: In an additional step to bring more nutrients to the algae, the interface between air and water will strip the “boundary layer” away from the algae, allowing the most nutrients to get to the algae. The thinner the layer of water, the better, all the way down to the “zero” thickness of air bubbles which physically rub the algae. Only scrubbers use an air/water interface: Waterfall and horizontal river style scrubbers use flowing water, and bubble upflow designs use air bubbles. Further reading of this can be found in the “Dynamic Aquaria” book by Walter Adey.
Refugiums are easy and cheap if you already have a sump with space, so we’ll cover points about scrubbers and reactors.


Chateo Reactors vs. Algae Scrubbers

Scrubbers and reactors were created to overcome the limitations of growing macro in a fuge, and thus do a better job at removing nutrients faster, and in a smaller space. But the tradeoff is they must be built, or bought. So here is some comparisons between the two types:
  • Size: A chaeto reactor needs to be much larger than an algae scrubber, and this is why you see them standing tall in a sump. Many saltwater tanks have large sumps, and even dedicated fish rooms, so this may not be an issue. Through experiential results of individual aquarists running chaeto reactors over the last few years, and through many thousands of aquarists running algae scrubbers over the last ten years, it has been observed that a chaeto reactor needs to be 4 to 8 times the physical size of an algae scrubber to provide the same rate of filtering capacity (rate of nutrient removal). I personally use the number 5, as to how much larger a reactor needs to be than a scrubber.
  • Seeding: A chaeto reactor needs to be seeded with a small amount of chaeto, either from another aquarium, reactor, or from your last harvest (i.e., you don’t harvest all of it), whereas an algae scrubber will self-seed from invisible algal cells in the water. When self-seeding, algae scrubbers usually start out with a slime type of growth, and this sometimes progresses on to a green hair algae growth, depending on the nutrients in the water. All growth removes nutrients from the water.
  • Harvesting: For a chaeto reactor, you disassemble the reactor usually by unscrewing several screws on the top of the container, and then by pulling out a tube or frame from the container; the chaeto growth is then removed from the frame and the frame is replaced back into the container, and the lid and screws are put back into place. Since chaeto does not attach to a surface, you can get broken chaeto pieces that flow into your tank or sump when you harvest; a filter screen in the reactor can reduce this.

For an algae scrubber, cleaning (harvesting) a horizontal river design will have a light that you lift up off of the container, and a screen that you remove from the container; you then scrape the screen and replace it.
  • A waterfall design will have a screen that you remove from a pipe; sometimes the whole pipe is removed, and sometimes the pipe is in a container that you need to open first.
  • A bubble upflow design has at least part of the container under water, which you lift out of the water.

And for all algae scrubbers, since the growth is attached to a surface, broken floating algae pieces are not common when you harvest on a proper schedule. Bubble upflow scrubber growth almost never detaches because the growth is always supported by the water. Also for all scrubbers, and depending on the design, it’s sometimes possible to just reach in and grab some GHA growth and pull it off, essentially making the harvest time zero.
  • Feeding: Feeding your fish from the growth lets the fish eat naturally, and you don’t have to buy and add food to the water (which creates more nutrients). Very few if any aquarium animals eat chaeto, but most eat green hair algae. The growth can be harvested and given to the fish by hand, or the filter can be opened and placed in the water for the fish to eat from. If it’s a bubble upflow type of scrubber, sometimes there will be enough flow coming out of the circulation holes that the fish will learn to just eat from there. Or of course you can just open the unit and let the fish inside (turn the growth lights off first, however).
  • Overgrowth: A last difference is overgrowth of algae on the algae itself. Chaeto is a slow growing species of algae because of it’s thick cellulose structure, and if conditions favor faster growing algae, you will get green hair algae which attaches on top of the chaeto, causing the chaeto to be blocked from light and flow, and eventually causing the chaeto to die and rot. There is no easy way to wipe green hair algae from chaeto; the chaeto must just be harvested earlier instead. For algae scrubbers, green hair algal growth on top of more green hair growth is how scrubbers operate in the first place, so earlier harvesting is not needed.
  • Nutrients: These are “bad” and are defined as inorganics, not organics. The word "nutrient" is sometimes confused with "nutrition", and in restaurants the words might mean the same thing, but for aquarists they are totally different. Organic nutrition food particles are mostly visible, but inorganic nutrients are invisible, and for aquariums these nutrients are:
  • Ammonia/ammonium
  • Urea (pee)
  • Nitrite
  • Nitrate
  • Phosphate
  • Iron
  • CO2
  • Organics: These are food particles, and most of them are big enough to see (and have nothing to do with “organic” foods in the grocery). They can be apples, pellets, nori, baby brine shrimp, flakes, peanut butter, poop, mucus, leaves, twigs, fish eggs, and other types of detritus, all of which are eaten/consumed by some type of organism. Organics are usually very visible when concentrated, and only after the organics get digested by a long chain of animals and bacteria do organics become invisible inorganics (this is called “remineralisation”, because they are now basic minerals once again). Organic food particles (which include detritus waste) can be large, small, or dissolved, and if dissolved in water then the water may be cloudy or clear. For example, if you take mucus (coral food) and blend it in water, the resulting dissolved organics would be invisible. Natural reefs are very highly loaded with organics, especially at night; sometimes a night diver cannot see his own hand because of the camera light reflecting off of the thick mass of particles. Lakes even more so; sometimes you cannot see more than a meter underwater in full daylight. Aquarium keepers however tend to want unnaturally “clear” water, where all the natural food particles have been removed. What algal filtration does for you, is allow you to remove all the bad inorganic things from the water, without removing the good organic food particles.
  • Alkalinity: Alkalinity and CO2 are commonly dealt with in aquariums, but their relationship to reactors and scrubbers is not obvious until you know what to look for. Algae use carbon to build new cells, and like any organism, algae prefer to use the easiest source of it. For algae, the easiest source of carbon is dissolved CO2 in the water because the carbon is relatively easy to separate from the oxygen. However there is not much CO2 in the air in a room, and even less of that dissolves into the water very easily. The CO2 that does dissolve into the water does so very slowly. So if algae is growing (filtering) quickly, it starts consuming dissolved CO2 (to get the carbon) out of the water faster than CO2 dissolves into the water from the air in the room. Thus CO2, and more specifically carbon, becomes limiting and slows down the growth (filtering). So what’s does algae use for a backup plan when dissolved CO2 runs out? Alkalinity.

Alkalinity in aquariums comes from bicarbonate HCO3- and carbonate CO3-2 which both have carbon, but the carbon is harder for the algae to get than with just CO2. So if algae is growing (filtering) quickly it starts consuming Alk to get the carbon. Oceans, reefs and lakes have a very constant rate of algal growth (it’s pretty much always high) and a large supply of alk, so alk is already at a balancing point and its level is not affected changed much (more info here).

But aquariums can have very large increases in algal growth if a rapidly growing reactor or scrubber has lots of nutrients to consume; and of course, alk is limited. So changes in alk levels can become easily measurable in aquariums with strong algal filtering, and that’s where the difference between algal filters comes in. Chaeto reactors are built to keep air out, and thus the only dissolved CO2 that is available is that which is already dissolved. So a rapidly growing chaeto reactor will start consuming alk, sometimes as much as 1.0 points per day, and as the alk drops so does algal growth and filtering because carbon molecules are not available to the algal cells.


Scrubbers however utilize a turbulent air/water interface, and once dissolved CO2 in the water runs out, CO2 can then be taken from the air without using alk from the water. A horizontal flowing river scrubber design has less than 2 cm of water thickness, so some CO2 can get from the air in the room to the algae quick enough to help growth. A waterfall design has an even thinner sheet of water flowing downwards, and at a higher velocity, allowing even more CO2 to dissolve into the water more quickly. So waterfalls rarely get daily changing alk drops, even when growing quickly. Lastly, a bubble upflow design has practically zero water thickness between the bubbles and the algae when the bubbles flow upwards and rub the algae, because the inside of a bubble is dry. Thus there have been no reports of alk drops with bubble upflow designs, and thus these designs can have the highest efficiency of filtering for a given algal filter size. Refugiums, by comparison, don’t grow fast enough to alter alk.
  • Combining Filters: A last note about combining filter types: Since algae removes lots of nutrients from the water, when you clean/harvest the filter you get less algae afterwards which means less filtering. So it’s recommended to have multiple filters if possible; for example if you are going to choose between one bigger and two smaller units, get two smaller. This way you can harvest one and leave the other one to continue growing and filtering. Combining different types of filters, however, is not recommended; don’t put a reactor with a scrubber, or a scrubber with a fuge, etc, because the stronger one will out-compete the weaker one for nutrients and you will just end up feeding the weaker one to the stronger one. Generally, a scrubber is stronger than a reactor, and a reactor is stronger than a fuge.


Hopefully this basic information will help those who want to run a modern naturally-filtered reef tank.

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