What is that strange smell?

What to do when you detect something…fishy…in your aquarium

Just about anyone who has kept a marine aquarium has experienced an unexpected biological “meltdown” in their tank. Typical symptoms include a strange odors in the water, cloudy, disccolored water and in the worst cases, fish and invertebrates in distress. Coming home to a biological imbalance often causes panic, resulting in the aquarist reacting rather than carefully investigating the cause and determining how best to remedy the situation. We’ll take a look at common causes of aquarium crashes and how to handle them.

Strange Odors in the Saltwater Aquarium

Describing an odor is quite subjective. When people say they love the smell of the ocean, they are really smelling a cocktail of natural organic substances released by algae. Want proof? The next time you mix up a bucket of saltwater, smell it. Does it smell like the ocean? Nope. Now smell your reef tank. Chances are it will have a faint “seashore” odor that we associate with a diving trip or holiday at the shore. This is caused by the release of oils and sulfur compounds by the millions of microscopic algae living and dying  in the aquarium. This is completely natural and indicates a balanced marine aquarium. Other odors, however, indicate something is not quite right.

One of the most common odors encountered in the aquarium is hydrogen sulfide (H₂S). Alsoknown as rotten egg gas, hydrogen sulfide has a reputation for being deadly poison to fish and invertebrates. Before we set the record straight about the toxicity of hydrogen sulfide, we’ll take a look at where it comes from. If you have ever had a canister filter fail and opened it up, chances are you were hit in the face with a blast of H₂S. The gas came from decomposing organic matter trapped in the low-oxygen (anaerobic) environment, inside the broken filter. The decomposing organic matter used up the oxygen, allowing the anaerobic decay process to begin. Hydrogen sulfide is one of the gases of decomposition, having a most noxious odor. Hydrogen sulfide also forms inside the aquarium. Wherever debris can collect and decompose, some H₂S will form. While this natural process occurs mostly undetected, you may notice a bubble or two rise up when you move a piece of live rock. This bubble could also be harmless nitrogen gas or carbon dioxide. The interesting thing about H₂S is that it is toxic but rarely harms anything in the aquarium. Here’s why. Hydrogen sulfide can only exist in an anaerobic environment, like a stagnant canister filter or deep in the pores of live rock. As soon as the H₂S gas contacts oxygenated seawater, it is oxidized to a harmless form. It takes a lot of hydrogen sulfide to cause problems in the reef tank. If, however, you aquarium has a persistent rotten egg odor, you’ve got problems. Assuming your reef is established, having healthy cured live rock, there is probably a dead fish or invertebrate decomposing somewhere in the aquarium. Take a visual inventory for a missing fish or invert. Once you find it, carefully net or siphon out the remains. Test for ammonia and nitrite. If the tests read positive, make a water change and test again in 24 hours. The biological filter should take care of any residual ammonia within a day.

 Cloudy Water, Green Water, Yellow Water

Discolored aquarium water is caused by several different situations. If you find your reef aquarium to contain floating white stringy debris, it usually means something has decayed behind the rock. Look for a fish or invertebrate that is sloughing off decaying flesh. Use a net to capture as much as you can. Change your filter media once a day until the water clears up. Although rare, reef aquariums can experience a thick “green water” algae bloom. The green coloration is due to millions of floating green algae cells. For some reason a single species took over, causing a disruption in the natural algae balance. Water changes probably won’t help. Some aquarists turn off the light for a few days, to trigger a die off and reboot the algae balance. If this does not work, a small UV sterilizer will kill the floating algae and keep the bloom away. If your saltwater develops a yellowish tint, which is easy to see when making a water change in a white bucket, it signals a build-up of dissolved organics in the water. Our reef tanks are closed ecosystems. Natural organics build up constantly. Protein skimmers and activated carbon can help, but nothing refreshes the tank and removes organics like water changes. If you use chemical filter media, change it at least once a month. But don’t neglect the water changes!

The Aquarium Distress Signal

What if the only thing you see in your reef are fish gasping and invertebrates in trouble? The first rule is Don’t Panic! Now is the time for clear thinking. Let’s assume you did not accidentally add anything harmful to the aquarium (if you did, start making water changes immediately). The first thing to check is the heater and filtration system. If they are OK, test the pH, ammonia and nitrite levels. The idea is to illuminate the easiest and most obvious things first. If the water chemistry is incorrect you know to make water changes and look for something that died. Did someone pour a bottle of invertebrate food in the aquarium? You will have to figure that out yourself by checking your supply cabinet. If all this checks out, it could be a toxic algae problem. I have seen it and duplicated it in the lab. Certain strains of algae can release neurotoxins into the aquarium. Fish will become very stressed and sometimes die. You won’t be able to test for it or even know for sure. The best thing to do is make water changes. Whatever is in the aquarium water needs to be diluted. It is impossible to say how much water to change, but 20% is probably the minimum. If the fish begin to breathe normally you’ve most likely changed enough water. Once things have returned to normal it is time for detective work. Retrace your steps over the last 24 hours, reviewing who could have done what to the aquarium. In most cases you will never know the cause and it will probably never happen again.

Keep Calm and Change Water

The most important thing to remember in the midst of a biological meltdown is to remain calm and think things through. Take a logical approach, checking each possible cause and eliminating them one by one. Even if you are unsure of the root cause, water changes help each of these situations. As the old saying goes “Dilution is the solution to pollution.” This is especially true in aquariums.