By Bob Goemans
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Q&A - Cyano

Authored by: Rob Toonen

HELP! I've been battling a red algae outbreak in my marine tank for about a month. I've tried cutting back lighting to merely a nearby table lamp; at first the algae started to disappear but recently it seems to have gotten used to the low lighting level and has begun to flourish again. In defeat, I went back to burning the fluorescent tank light for 8 hours a day and now have a sea of red! I don't believe I have a water quality issue (a sparsely loaded 55 gal. tank - four small fish, a shrimp and a hermit crab). Ammonia, pH and nitrite levels are all at desired levels. I can't expect anything desirable to survive on less light. What can I try? Thanks.

Via the Internet


I usually don’t even try to answer questions such as this, because the number of potential causes and solutions to any problem is so great that it is impossible to do justice to your question in any single reply. Even if we had an hour to sit down and discuss this in detail, I wouldn’t necessarily guess the “right” answer for you immediately, because I simply don’t know enough about how your tank is set up and maintained to do anything but guess at solutions.

Having said that, however, I’ll give this a shot. There are several key pieces of information that you neglected to provide, and which I suspect are likely to be the ultimate source of your problem. First, you don’t tell me whether your tank is a reef or fish only tank, or what sort of filtration you provide. I’m going to guess that this is primarily a fish tank, and that the live rock, if any is included, is there more for decoration than filtration. I would also guess that means you’re using some sort of external or undergravel filter in the tank. The reason that this is an important detail is because these methods of filtration rely on removing organic particles from the water and allowing bacteria to decompose them. It’s generally obvious to everyone that fish need to take in nutrients and excrete wastes like any other animal, and although it is not always so obvious, it is equally true of every living organism on the planet from bacteria to humans. Animals wastes (whether fish or invertebrate) are primarily the nitrogenous compounds you mention above, ammonia (NH4) and nitrite (NO2). You’re obviously aware of the nitrogen cycle in an aquarium, and understand that these chemicals are highly toxic to most marine organisms and need to be kept low in order for your pets to survive. However, the breakdown of these toxic nitrogenous wastes by the bacteria in your filters is not complete – by that I mean the bacteria that you cultivate in your filter do not simply eat ammonia and nitrite without releasing some wastes themselves. The waste product of metabolism for these bacteria is primarily nitrate (NO3), which is in turn a nutrient for the growth of algae which ultimately release oxygen (O2) as their waste. Obviously this is a highly simplified view of the world, but it should get the point across – even our filters produce waste, nitrate, which although much less toxic than ammonia and nitrite, can still be toxic (especially to reef-dwelling invertebrates) if the levels rise high enough.

That brings me to my second point, and that is that you don’t tell me exactly what you mean by “desired levels” of ammonia, pH and nitrite (I would have preferred the actual measurements -- desired levels often means different things to different people), but as I just explained, these are unlikely to be the primary nutrients that fuel algal growth (although algae are certainly capable of using them). Because you don’t tell me what you nitrate level is in your tank, I must assume that you have not measured it recently, and I suggest you add this test kit to your arsenal for the future. Although there are ways to reduce nitrate levels in an aquarium, most are impractical or unnecessary in an aquarium dedicated to fish. Fish are generally much more tolerant of elevated concentrations of nitrate than are invertebrates (although there are exceptions to every such generalization), and levels of nitrate in fish-only tanks is usually 10 or 100 times that considered acceptable for a reef aquarium. Despite the fact that fish are generally more tolerant of this bacterial waste product, there are suggestions that elevated nitrate may play a role in some disorders of tropical fish, and if nothing else, the fact that it fuels algal growth is a good reason to try to keep the level low in even fish-only aquaria. One simple way to do that is to add an efficient protein skimmer to your aquarium – in its simplest sense, a protein skimmer functions by removing some of the fish waste before it can be converted to nitrate by the bacteria in your filters. Obviously, if you look through the pages of any issue of TFH, you’ll see there are many different kinds, shapes and sizes available to fit any aquarium design.

However, nitrate is not the only nutrient (or “fertilizer” as most people think of plant foods) required for algae to thrive. Algae also need phosphate (PO4) to grow well, and again, I would consider adding this test kit to your supplies. Phosphate is introduced through foods and potentially detergents, but the most common source is almost certainly water changes. Depending on your location, you may be able to find out the composition of your local water from your local government, or you can always test the amount of nitrate and phosphate in the tap water yourself. Using a salt that does not contain nitrates or phosphates, and purified water (reverse osmosis, deionized or distilled) will ensure that you are not adding substantial amounts of algal nutrients with your water changes. Saturating any water used to make up for evaporation with calcium hydroxide (CaOH, otherwise known as kalkwasser) will remove most of the phosphate as a white precipitate (you must allow this powder to settle out before removing the clear liquid above it to add to your tank) and seems to increase the efficiency of protein skimming, both of which helps to reduce the concentration of algal nutrients in the tank.

In tanks in which both nitrate and phosphate are available in abundance, a bloom of the dreaded hair algae (such Derbesia) usually follows soon after. However, you don’t mention hair algae in your question, but rather refer to “red algae.” I suspect that what you’re seeing is not a red alga at all, but rather a cyanobacteria – better known as “slime” or “blue-green algae.” These cyanobacteria are common in new tanks, tanks in which the light levels are low, but nutrients are high, or those in which the nitrogen levels are kept under control, but the phosphate levels remain high (this is because cyanobacteria are one of the few capable of providing their own nitrogen source). Like your nitrate levels, you don’t tell me what your phosphate levels in the tank are, but I suspect that they’ll be elevated. Also, you don’t mention what kind or how many lights you have on your tank. A single 40W flourescent bulb on a 55 gallon tank is quite low light, and again, this favors the growth of cyanobacteria over other algae.

I suspect that all of these factors may play a role in your problem. You don’t mention how long your tank has been established, but it is very common for tanks that have just recently been established (first few months) to go through a period of high cyanobacterial growth. This ugly slime is a common problem in many tanks, and although there are some “quick fixes” possible (certain medications), these are ultimately detrimental to most aquaria because you simply fix a symptom of the larger problem, you do not resolve its cause. The best way to deal with this red slime is to simply siphon it off on a regular basis, and make sure that your replacement water is of high quality and free of the nutrients that feeds these cyanobacteria.

There are a variety of things you can do to try to bring problem algae under control. First, if your tank is fairly new, it may actually come under control on its own, but I wouldn’t count on that solution, and simply don’t know how likely it is that this is just part of the cycling of your aquarium from the information here. The second option is to try to introduce some more desirable forms of algae (various species of Caulerpa are among the most popular and widely available) and increase the lighting to favor the growth of these more attractive algae in your tank. This sometimes works, but in my experience, people often find that the nuisance slime algae simply overgrows the macroalgae and you end up in the same place that you started. The final, and I think best, option is to control the input and/or increase the export of nutrients from your aquarium. There are many options here again. As I mentioned earlier, an efficient protein skimmer will certainly help you to export more nutrients from your system. There are also chemical means of removing nitrate and phosphate from marine aquaria (so-called chemical “sponges” or “pillows”). Regardless of which (or how many) of these options you decide to try, algal growth is ultimately linked to control of nutrients, and the bottom line is that if you can limit the amount of nutrients added to an aquarium, while increasing the amount that is removed, you will eventually succeed in curbing the growth of this unsightly slime algae.


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