Vinegar Eels

Jim Atchison
The Bug Farm

Talk about an easy food to culture! If you have too much trouble with this food, you probably kill grass on a spring day.

There is debate over the value of this food only because of the perceived difficulty in harvesting the little things. Well, read on and you decide.

While we call them eels, we are really talking about nematodes. The word nematode literally translated means "thread-like". In Great Britain and other parts of the World, the commonly used term is "eelworm" and you may encounter this term in reference material.1  

The nematode in question, commonly referred to as a "vinegar eel," has a proper scientific name: Turbatrix aceti or tubatrix aceti, depending on which published and credible document you read. The University of California, Davis1, refers to the organism as from a genus Turbatrix, while the University of Wisconsin, Madison, 2 refers to the genus as Tubatrix. I don’t know about you, but I’m going to stand back and let the folks with the federal grant money do battle and settle on a single name for the genus.

Nematodes have been around a long time. While Nematodes have left very little direct evidence in the form of nematode fossils (I’m sure that would be a frustrating collection to come by), some nematodes have been found preserved in insects within 120-135 million year old amber. Ancient writing references nematodes in China some 4,690 years ago with the recording of intestinal roundworms.1

But it was the development of the microscope that made the study of small creatures possible and widely practiced. The availability of the microscope brought about increased interest in the smaller, free-living nematodes and their structure.1 Some of the now famous scientists of their day made discoveries using microscopes. Some studied other creatures as well but, Hooke, Baker, Leeuwenhoek and Spallanzani all studied free-living nematodes. It was however, Borellus who identified Turbatrix acetia in 1656.1 To be classified a Nematode the creature must have some common characteristics. Nematodes are un-segmented roundworms, usually elongate and tapered at both ends, bilaterally symmetrical, and with a complete digestive tract. Most show considerable sexual dimorphism, with the female usually larger and the tail of the male being more curled.2

Juvenile nematodes require several molts before the adult stage is reached, and development may be either direct or may require an intermediate host.2

YIKES! What’s this talk about an intermediate host? Well some nematodes are parasitic and some are not. Vinegar eels are NOT parasitic and NO NOT need an intermediate host. Vinegar eels are what are referred to as a "free living" nematode.

You may be asking, "Why vinegar eels and not some other food?"

We have found that the lowly vinegar eel has some advantages over other similarly sized foods. The most commonly cultured food of a similar size (vinegar eels have a maximum size of about 1/16th of an inch , or about 2 mm) is the microworm (Panagrellus sp. or Anguillula edivivu (silusiae). We use this food as a transitional food from greenwater and infusoria into baby brine shrimp and Microworms. However, the microworm, while easy to culture and harvest falls through the water column and then rests on the bottom or substrate in the tank. Microworms do not swim through the water. Unless you are feeding a bottom dwelling fry (Corydoras being an excellent example of such a fish), the fry may not find the microworms to eat them, or at least may have had a more difficult time finding them. Vinegar eels are easy to culture. So easy that sometimes it is easy to forget you have a culture available. This ease of culturing is a tremendous advantage for the aquarist who might not be planning on their fish to spawn or did not’t realize that the larvae that just hatched are too small to eat baby brine shrimp when the fry hatch. In any event where one might find themselves a little on the unprepared side (it happens), vinegar eels can fill the void. Vinegar eels swim throughout the water column. While microworms may fall through the column and rest on the bottom, vinegar eels do not. They swim throughout the entire water column making them more available to a wider variety of fish larvae.

Vinegar eels can survive for days in water. Most live foods fed to fish are not truly aquatic and can die within hours of their introduction into the tank. Granted, while some "bugs" can live for several hours, most need to be eaten within the day. Vinegar eels however, can live in a water solution for days on end. The environment of the tank is not going to encourage the eels to propagate, as they prefer a highly acidic environment, but survival until they are eaten is very likely. We have found that a single feeding of vinegar eels in the morning can easily furnish a tank of fry with food for the entire day. Because the living food does not decompose one does not have to underfeed or feed multiple times during the day to compensate for the decomposition of the dry foods or foods that die more rapidly. Remember too that you are using these small eels on small fish and while 10 eels may not seem too much to you, to the small fish it's a huge meal.

Much is said and written about the ease with which one is able to culture vinegar eels. The process is so easy that people frequently question the process. The old saying, "If it sounds too good too be true, it probably is," does not apply to vinegar eels. The process sounds too good and it is.

The first step in the culturing process is to decide how much culture you will need to be working with. We find that two gallons of culture will more than suffice for a "normal" spawn of Betta splendens. Because of the way we harvest the eels, by the time the cultures are all harvested, the fry in the spawn will be large enough to take other foods. Some folks will use less culture, but with the ease of culturing, having more is not too much of an issue.

After one has settled on an appropriate size of culture to work with, acquiring a suitable vessel is the next task to accomplish. We use one-gallon plastic milk jugs and gallon sized vinegar jugs for our culturing containers. One can use glass, but the possibility of breaking the glass is real. We find that plastic is cheap and plastic is readily available. If one is starting with a small "starter" culture, begin by putting together one, one gallon culture and when that culture is mature, use a starter from your first and now mature culture to start a second appropriately sized culture. Each of your cultures will need to be covered. The purpose of the cover is to keep flies and gnats out of the culture. The eels will not escape an uncovered vessel. The cover should be such that air can pass through the material yet bugs cannot infest the culture. Cheesecloth works, flour sacking works and foam plugs work on the narrow opening of gallon jugs.

The ingredients for the culture medium are easy to remember, easy to acquire and easy to mix together. You will be able to find all of the ingredients at your local grocery store. In the one-gallon container we add of an apple. We frequently joke about using "Fuji" apples because we like to eat the second of the apple and we like Fuji apples, but any type of apple does just fine. You do not need to peel, nor do you need to core the apple. After we place the apple into the container, we fill the container about 1/3 full of tap water. The water does not need to be de-chlorinated. We use cool water, not warm or hot. We then add enough apple cider vinegar to the container to bring the surface of the culture to about of the way up the taper at the top of the jug. You really cannot use regular vinegar, you need to use apple cider vinegar, it’s both for the sugar in the apple cider and the acid in the vinegar. The ratio of water to apple cider vinegar is approximately 1:1 (or 50/50). The container should not be completely full, but neither would it be filled to the shoulder of the jug. One wants to allow a pretty significant surface area to insure that the eels will get enough oxygen. We then add the starter to the mixture, cover the container and put the whole mess aside for a month. Essentially you are growing bacteria in the decomposing apples and the cider, on which the nematodes will feed.

You can store the culture in a semi-dark place (the garage is good for most folks), in a normally lit room (the kitchen counter worked for us for years), or you can put the culture in a dark location (under the kitchen counter worked after all those many years). While they can be cultured in a much wider range, their ideal range for propagation is about 75-85 degrees F (25-30 degrees). We have grown them in conditions from 40F to 105F for short periods of time, but sustained temperature outside of their ideal range will effect the production of the eels. They do have a tendency to smell a little bit like a salad dressing. Family politics keeps ours out of the house. However, generally speaking, vinegar eels are very forgiving in regards to their environmental needs.

The culture will be ready to harvest in a month. During the month of waiting, you may notice fungus and/or mold growing on the top of the container. Neither the fungus nor the mold will create a great deal of effect on the culture. You may notice that the apple breaks up into a pulp, or they may stay whole. The texture of the apple will not affect the culture and is strictly a function of the type of apple you start with. The bottom of the culture will develop a somewhat large amount of detritus over time (mulm, guck, "stuff", junk; you can call it lots of things). The detritus will in part be the apple but will also contain precipitate from the vinegar along with the molted skins of developing eels. You do not have to remove the material unless it becomes so deep as to affect the culture. We have not found it to be a problem unless the culture is about filled with the material. You might notice that the culture looses some volume to evaporation. You do not have to compensate for the evaporation during that first month. It the adage, "A watched pot never boils," is true, leave the culture alone and try not to think about it too much. Wait a month and then see what you have to work with.

At the end of a month, remove the culture from the hiding place. Seeing the actual eels is a little tricky, but they can be seen with the naked eye. It helps to use a 10x magnifying lens, but one can see the motion of the eels by simply placing a portion of the culture into a clear container (your spouses favorite glass in not recommended, but it works) and holding the culture up to a strongly lit window. "Back-lighting" the culture with a flashlight works also. The eels will look like shimmering slivers of glass within the liquid. You will probably have a little difficulty seeing them, but remember to look for the shimmering and to use a very bring light behind the culture.

After the one-month has passed, there are a couple of options. If one does not need to feed any fish at the time, one can start a second culture with a small portion of the first. If there has been some fluid loss from evaporation, the vessel can be topped off with regular tap water. If one is ready to feed to fish, harvesting the eels is now possible.

Harvesting can be the tricky part of the vinegar eel culturing process. Some people put a small piece of open pored filter material directly into the culture and remove it after a few hours, then swishing it into a tank of fry. Other folks pour the culture through a coffee filter, rinsing the eels before swishing them into the fry tank. Either of these methods will work. There is some concern for the addition of any culture media to the fry tank. While that addition may be an issue if too much material is added to the tank, if one is cautious there should be not problem. From the coffee filter process, one would want to capture the media that flows from the filter so that it can be replaced into the mother culture. Both of these methods will caused the aquarist to acquire the faint aroma of a salad dressing, as it is nearly impossible to keep the material from getting on ones hands. 

We use a method of harvest that is modeled after a common procedure used in Europe. Where the original model used a test tube filled with culture, stoppered and topped off with water, we have found that the same principle works with larger containers. Using the eel’s own biological need for oxygen, we take a long necked vessel (fancy vinegar bottles work well as do some salad dressing containers), fill the body of the vessel with culture, push a wad of filter floss (or polyester pillow batting) down into the neck of the vessel until the wad touches the culture and then top off the neck of the vessel with tap water. The eels, in their quest for oxygen, will slip through the stopper and into the clear water, leaving the culture media behind. Removing the eels with an eyedropper is easy. They can be fed directly into the tank of fry with little residue from the culture media. 

The only down side of the stopper method of harvest is that the technique is so effective that virtually all eels are remove in a couple of day. We have also found that the vast majority of eels are removed in the first 24 hours or so. Because of the effectiveness, we have found the harvest rhythm that works best for use is to assemble the harvest system in the evening in anticipation of two harvests the following day. We remove the culture media from the vessel after the second harvest, placing the spent medium into a container to rest (and rebuild), and then re-assemble the system with fresh culture media for the next day. It may take a month for the spent culture to rebuild. It will take longer if one leaves the media in the harvest system for longer than a day as one can remove enough eels to render the culture barren.

We used to use a small baster and drew some eel laden fluid up and transfer it into a funnel with a coffee filter inside it. When we thought we had enough eels (generally two basters full), we waited a couple of minutes to let the fluid drain back into the mother culture. We then ran a gentle stream of cold fresh water through the same coffee filter/funnel contraption and let them drain a bit. When the water had drained, we repeated the washing routine. We next turned the filter inside out and swish the inverted filter into a cup of water. We generally had a number of tanks to feed so we tended to swish the filter into a beaker of water and using an eyedropper, feed from the beaker of eels.

Because of the time waiting for fluids to drain, some people have developed alternates methods of harvest as we described a little earlier. At the time we used the filter paper method, we found that the waiting did little to slow down the preparation of the fish's meals. We simply used the waiting time to prep other types of foods. All issues considered, the long necked bottle method is more efficient than the filter paper method.

We find that vinegar eels provide a tremendous advantage with numerous species of fishes. We use them with nearly all egg layers, including many of the New World cichlids. Frequently we find that a batch of fry will have wide variations in the growth rates amongst the clutch, feeding a much smaller food that the average fry might eat seems to increase the number of fry which can survive. Even with fry that can eat baby brine shrimp at hatching, we tend to use vinegar eels to bridge the period between feedings. Many fish only eat in a particular zone of the water column. Many of the rainbow fishes eat primarily in the top sections of the tank. Vinegar eels, because of their ability and tendency to swim throughout the entire water column can be reached throughout the day by most fry. While we doubt that the eels is as nutritious as newly hatched brine shrimp, we are equally convince that any food is better than no food and some fish simply need a small food such as vinegar eels.

These little nematodes are a great subject for science fair projects. We have worked with dozens of young scientist in their quest for the perfect answer to questions such as, "how much vinegar do the eels need?" The answers to what will increase the productivity and the nutritional value of vinegar eels vary widely. Most of the information that is available regarding these nematodes is either very scientific or very kitchenific (a new word to represent the little-of-this-and-a-little-of-that techniques employed by most hobbyists). Someplace in between is a happy balance between the science of biology and the art of breeding fish. 

(1) The Internet: History (
University of California, Davis, CA.

(2) The Internet: The Parasitology Resource  (
University of Wisconsin, Madison, WI.

Reprint Policy: While this document is copyrighted, the author encourages reprinting of the material for non-commercial purposes and in it's entirety, as the practice may help hobbyists in their effort to captive spawn more fishes. When reprinting the material, please give consideration to the author, cite the document properly and send a copy of the publication (or it’s location) to the author. Information on contacting the author can be found on the web site:

  J. Atchison, 2003, 2009