Controlling Fungus on Fish Eggs

By Dwight D, Moody

 TFCB Summer, 2014

All of us that breed fish eventually have to deal with fungus on fish eggs. We feed our breeders lots of live foods to get them into condition, set up the right conditions for breeding, they start acting like they are interested and VOILA! We have eggs. We wait for them to hatch and then they start getting furry Ė the dreaded fish egg fungus. Maybe some hatch but we lose a lot of eggs to fungus, sometimes the entire batch is a total loss. Frustrated, we wonder if there is any way we can save our precious spawns from the dreaded egg fungus. Fortunately, there are several methods that can be helpful.

The first method is to make sure that we actually have males and females in the breeding group that actually lay and fertilize eggs. Sometimes, a supposedly fertile pair turns out to be two females. Other times, for various reasons, either the eggs were not fertile or were not fertilized by the male, who may not have been mature enough to spawn. Other times, the couple may spawn and then they eat the eggs. Sometimes this means that the eggs were not fertilized, sometimes the couple is just inexperienced. In some cases, ensuring there are a multiple mating choices for each female can ensure a successful spawning. In other cases, such as with Corydoras catfish, having multiple males for each female seems to be necessary for success, because the multiple males ensure the female is properly stimulated plus there are multiple opportunities to ensure that a fertile male is involved in the spawning activities.

Once you have a successful spawning with fertile eggs, the next problem is to prevent fungus from attacking the eggs. Some fish will take care of this by regularly cleaning the eggs as well as fanning the eggs to ensure adequate water flow, both of which tend to limit the ability of fungus to get established on the surface of the eggs. In other fish, such as tetras, Corydoras catfish, etc., the parents provide no care of the eggs. Often, the eggs are laid where there is sufficient water flow to prevent fungus, but leaving them in the aquarium may not be possible. I have found that fungus is particularly common in water with alkaline pH, and fungus does not like low pH environments. So a good strategy to prevent fungus is to lower the pH of the breeding tank. This can easily be done with aquarium chemicals that lower pH, or you can use one of the Blackwater Extract liquids to accomplish the same thing, which has the added advantage of adding organic acids that may further inhibit fungus growth.

If your fish and eggs need to be maintained at an alkaline pH, there are several commercially available chemicals that may help. The most commonly used and traditional anti-fungus aid is methylene blue. Methylene blue is fairly good at preventing egg fungus, but it has issues associated with its use. First, it stains EVERYTHING it touches, so you have to be very careful not to get it on anything you donít want stained deep blue, which includes hands, clothes, etc. Secondly, it also tends to wipe out your biological filter, so you can save eggs just to have your fry die of ammonia poisoning. If possible, use the Methylene Blue as a dip, then rinse off the excess. It will stain the egg shell but should not affect the rest of the tank much. Methylene Blue can be obtained from Kordon (

Another alternative egg treatment is Acroflavine. This has the advantage of having anti-fungal effects and is especially effective at higher pH levels. It is known to not be good for plants. Like Methylene Blue, it will affect the biological filter but is easily removed by activated carbon. It may be used as a dip or a short-term, temporary treatment, just remove the filter media, add the Acroflavine, remove it either just before or just after the eggs hatch with fresh activated carbon for three days, then add back in the old filter media which will restore the biological filter to prevent fry loss due to ammonia buildup. Acroflavine can be purchased online at AquaBid (, National Fish Pharmaceuticals ( or in a small dropper type bottle from Pet Solutions (, the latter option being the lowest cost option for those who only need a limited amount of Acroflavine.

Other chemicals that may be used for fungus control are Seachemís Paraguard at ľ dose (, Mardelís Maroxy, which is a sodium chlorite solution, which may be used at just a few drops per 20 gallons. Pimafix has also been cited by online users as another solution that may be tried, which has the advantage of being a natural tincture rather than a manufactured chemical. All of these solutions can be obtained at local fish stores, PetSmart, Petco and online retailers.

In conclusion, the best way to control fungus is to create a lower pH environment, when possible and feasible to do so. The next best solution is to use a chemical dip coupled with gentle water movement. Finally, use a chemical that affects the whole tank but remove the filter media, put it into another tank to keep it active, then use activated carbon to remove the chemical just before or just after hatching, then re-install the original filter media after all the chemicals have been removed to restore the biological filter and prevent fry loss due to ammonia poisoning. By using one of these methods, you will be able to control egg fungus and ensure that your spawns have a good percentage of eggs that hatch. Good luck, may your spawns result in you having lots of fry to raise and share with other fish club members!