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Overconfidence in the Tropical Fish Hobbyist

by TFH Magazine on January 23, 2012 at 1:01 pm


Skin flukes (Gyrodactylus spp.). Photograph by Craig Adams.

By Craig Adams

“I am God’s gift to koi,” my client began, “or so I thought, until this year.” We were standing at the edge of this gentleman’s pond in late June, who was having unexplained losses and was frustrated in his effort to keep his population of koi healthy. He ultimately lost about half of his large collection before the situation was brought under control. My client has had a 20,000-gallon pond for 20 years with a beautiful collection of koi, many of which were spawned in that very pond. Being a very advanced hobbyist, he even has an 8,000-gallon tropical tank in his yard in western Washingtonthat includes an amazing assortment of large tankbusters, such as Arapaima gigas, redtail catfish (Phractocephalus hemioliopterus), and Colossoma macropomum. He understands water quality, nutrition, and many of the intricacies of husbandry that allow him to be an ultra-successful hobbyist. The koi spawned and raised in his pond were like his children, so thinning their population to an appropriate level was difficult for him to do. Eventually, it caught up with him.

White Noise

Overconfidence can sometimes be damaging because it clouds our judgment and prevents us from seeing what is in front of us. This happened to me recently at home as well. My wife and I were trying to transition our baby girl from sleeping in a cradle in our bedroom to sleeping in the crib in her room. We started the transition by having her nap in the new crib during the day. We ran into trouble, however, because she seemed to be more aware of the everyday sounds around her and had trouble sleeping.

My wife thought that adding some white noise to her surroundings might help to drown out the other noises and let her sleep. Some people use the vacuum cleaner for white noise. There is even a white-noise machine on the market that sounds a bit like the static we used to get from the television in the pre-cable days when we could not get reception. That seemed like a colossal waste of resources to me. It did not take me long to come up with the perfect white-noise generator. I grabbed an empty 20-gallon tank from the workshop next to my fishroom and set it up in the baby’s room. Naturally, this gave me a great excuse to buy some new fish to stock it.

We do not have a large selection of local fish stores in our area, at least for freshwater fish. Since this was an emergency (I certainly could not generate white noise with an aquarium that was devoid of fish), I headed to the local outlet of a nationwide distributor. I chose two of my all-time favorites: angelfish and Corydoras catfish. My wife selected a small school of each species, a plastic sea turtle, and an artificial Amazon sword plant. We were now prepared to convert our baby’s nursery into a white-noise paradise. She would no longer be jolted awake by cars honking, dogs barking, or me tiptoeing down the carpeted hallway while holding my breath.

Once we got them home, the cories quickly began exploring their surroundings and rooting through the sand substrate looking for food. The angelfish established their pecking order and chose spots along the plant and plastic turtle while keeping a keen eye out for the bits of food that magically appeared on the water’s surface a couple of times a day. This was, for all practical purposes, an Amazonian biotope setup from the plastic sea-turtle region of South America. Ahh, fishkeeping nirvana.


Unfortunately, all was not as it seemed—I lost an angelfish on the following day. This is not unusual with a new batch of fish. Just the act of netting fish and transporting them to a new tank can cause stress, elevated cortisol levels, and subsequent reduced immunity. Transport losses are expected to a certain degree, but never welcome. Did I quarantine these fish? No. This was a new setup, but the filter was already mature since I pulled the foam insert from another tank I already had running. I was therefore treating the entire system as a quarantine tank. I would not have thought of putting these fish directly into any of my other tanks that already had fish in them.

Over the course of the next few days, I slowly lost fish, both cories and angelfish. My wife chided me a bit since the big fish doctor was losing fish in his own daughter’s room. I tested the water on a daily basis, as should be done on all new tanks. The filter was functional, and I never recorded ammonia or nitrite spikes. I chalked the losses up to ongoing stress following the move from the store. One afternoon after changing a diaper, I was watching the fish with my daughter. By finally paying close attention, I could see that one of the angelfish was hanging back a little while the others were actively feeding. I decided that enough was enough. I got some of my equipment and performed a skinscraping on that fish—what I found shocked me.


I was dealing with one of the worst Gyrodactylus infestations I had seen for a long time. These are sometimes called skin flukes. There is a similar fluke, Dactylogyrus, which is also known as a gill fluke. They have different life cycles and can be found on the skin or the gills. Gyrodactylus flukes are viviparous (livebearing) while Dactylogyrus flukes are oviparous and have eggs that settle on the substrate to hatch prior to attaching to a host. Gyrodactylids attach to a fish with large anchors present in the opisthaptor (attachment organ). They are quite prolific, and in warm conditions, a population of flukes can double in 24 hours.

Once attached, they scrape mucus and epithelial cells from the host. This leads to irritation, excessive mucus production, secondary infections, and even respiratory distress if they are on the gills. They can quickly kill fish. One easy way to tell if the fluke is a gyrodactylid or a dactylogyrid is to look for the presence of an embryo in the fluke. The hooks on the embryo are often visible. These are microscopic parasites, so you will not see them with the naked eye. Excessive mucus on a fish is a clue, but it is not enough to make a diagnosis of a fluke infestation. Both of these are monogeneans—they do not require an intermediate host as part of their lifecycle. Digeneans, in contrast, are flukes that do require an intermediate host, such as a snail or a worm.


Well, once I had a diagnosis, I felt a little foolish. I had been operating under the false assumption that the problem was just stress related. Stress certainly could have played a role, but ignoring the problem would not make it go away. I added 2.5 ppt pickling salt (sodium chloride without anticaking agents) and 15 ppm formalin every other day with 50-percent water changes on the off days for three treatments. The salt was added to help reduce the osmotic stress that the fish were experiencing. Formalin is an aqueous solution of formaldehyde gas. As you can imagine, it is a dangerous substance to deal with if not handled properly. I happened to have some on hand since it is an approved medication for some food-fish diseases, so I decided to use it.

Most of you will not have it at home, and you would not know where to get it if you needed it. There is, however, a much safer alternative called praziquantel, which is a medication used to treat tapeworms and other parasites. There is a good chance that if you have a dog or a cat, your veterinarian has used it on your pet. Many of my fish patients live in ponds, and treatment with praziquantel can be very costly when such a large volume is needed, but treating an aquarium (even a large one) can be done for a reasonable price, usually less than the cost of replacing all the livestock.


Like a contractor with a home that has an unfinished basement or a painter with psychedelic wallpaper from 1970, the fish vet can have losses in his own tank. Once we step back, analyze the problem, and determine what is going on, the proper treatment becomes less of a mystery. Wait-and-see is not usually the best course of action. Most veterinarians could diagnose a fluke infestation in a fish whether they know it or not, but few veterinarians will travel to your house to make the diagnosis (although a growing number of us will). If you have a very sick or freshly dead fish and suspect flukes, you can put it into a bag for the veterinarian to look at in the office. Once you have a diagnosis, a treatment plan can be arranged to try and save the rest of the fish in your system.

Just adding one of these new fish to any of my existing aquariums could have led to the death of many of the fish already in my collection. That would not look good for a fish doctor. Always remember to quarantine. Your fish will thank you for quarantining, and they may even start to refer to you as “God’s gift to fish”!

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Posted in TFH Extras by TFH Magazine on January 23rd, 2012 at 1:01 pm.

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