Dealing with egg ‘fungus’


There’s nothing worse than having a spawn of eggs blighted by the appearance of fuzz. Here’s how to deal with a pernicious problem that besets many breeders.

Any fishkeeper who blissfully wanders into the world of fish breeding inevitably finds themselves at odds with a stubborn foe. The hairy growth on some eggs may seem innocuous at first but can rapidly overtake an entire clutch, putting a hard reset on whatever plans you had for your breeding project. The fuzzy menace is often dubbed a fungus, but is it really? And why does it seem to be such a universal problem during the earliest life stages of our fishes?

If you were to page through older books and magazines — indeed, you might find this on more recent websites too — you’ll find references to fungus in the aquarium and how to deal with it. However the most common agent we’re fighting is not a fungus at all, but rather an oomycete, otherwise known as a water mould. The most common pests in this category are Saprolegnia, having an appearance of soft, cotton-like growths where they’ve established. Saprolegnia typically subsists on dead organic matter, but will opportunistically feed on fish and their eggs where circumstances allow.

After water moulds, several species of bacteria are the next group of concern, although usually these take hold in the presence of other pathogens or poor environmental conditions. Most of these are harmless, beneficial even. However, if opportunity allows it, these organisms will jump on at the first chance to feast on vulnerable organic material. The presence of water mould damages the chorion, the outer egg layer, allowing these other nasties to take hold and further worsen the situation.

How it takes hold

Saprolegnia has a difficult time colonizing live eggs, which have some degree of immunity and built-in antimicrobial enzymes. However, these defences are limited and can easily be overwhelmed. Unfertilized, damaged, and dead embryos offer no resistance, and will readily succumb to Saprolegnia. Once established on deceased eggs, the water mould will move onto the adjacent healthy eggs.

Certain conditions can help Saprolegnia along, however. A low oxygen environment may lead to egg death, granting the pathogen new sites to take hold of. Eggs regularly find their way into sites with poor circulation, since they’re usually deposited in safer places out of sight and reach of any potential predators.

As such, it is common practice among some breeders to run an airstone not too far from the developing brood. This keeps the water moving and agitates the surface, enhancing gas exchange. Similarly, some breeders like to keep their eggs in the shallows, either in a container near the tank surface or in a separate, shallow dish. The proximity to the surface allows more efficient gas exchange than deeper water might allow for.

Unfertilised eggs will readily fuzz up since they’re basically balls of unprotected nutrients ripe for the picking. Infertile eggs are not uncommon among breeding fishes — even healthy animals can miss out the odd egg, be inexperienced, or in a worst case scenario, have fertility issues. Any of these factors working alone or together may contribute to a fair percentage of eggs being unfertilised, putting the rest of the batch at risk.

Those opportunistic microorganisms I spoke about earlier can certainly be problematic in some instances, but there’s a lot the aquarist can do to make matters worse. A poorly maintained tank — one that has allowed detritus to build up, leftover food to rot, and nutrients levels to climb — is a breeding ground for all manner of opportunists.

Again, these organisms are not typically harmful, but in such prime conditions their numbers can escalate. In vast numbers, it is very easy for these critters to overwhelm the vulnerable egg and fry stages of fishes. These very same conditions often go hand-in-hand with other stressors, like low oxygen, and can make breeding a whole lot more difficult than it needs to be. Keeping up with good tank maintenance helps to prevent this among many other problems typically associated with rearing fry through.

Prevention and Treatment

As per the adage, prevention trumps cure. It’s worthwhile remembering that you cannot completely prevent Saprolegnia from entering your aquarium. Their spores are abundant and hardy, and invariably exist in most aquatic environments. However, would-be breeders who can get in front of this pathogen before it takes hold will avoid unnecessary woe.

Whether or not you’ve applied a treatment prophylactically, keep an eye out for unfertilised eggs. These will be a different shade from developing eggs, although specific colours may vary from species to species. These eggs do not have even that little immune capability that fertilized eggs would do — rather, they’re nothing more than a clump of nutrients. Not only can they not fend off would-be pathogens, but they attract all manner of heterotrophic organisms, such as bacteria as well as Saprolegnia. In great enough numbers, these can overwhelm the defences and subsequently smother adjacent eggs, fertilised or not.

If a bit of fuzz has already begun to appear, don’t worry — you still have the opportunity to save most of your batch. Unfortunately, the eggs which already have a healthy layer of fuzz are beyond saving. It is very important to remove these affected individuals as soon as possible, for a few reasons.

The established hyphal mat can readily spread across to nearby, otherwise healthy eggs. This is due to the established water mould overwhelming the healthy eggs and lowering gaseous exchange and effective water flow to neighbouring eggs, facilitating egg death. Secondly, a more established fuzz layer can produce subsequently more spores, which may affect distant eggs to which the spores are attracted.

Excessive fuzz can also make it difficult to visually differentiate affected from unaffected eggs. In cases like these I’m happy to remove the eggs immediately adjacent to fuzzy eggs, healthy or not. There is a good chance that Saprolegnia has already got a small foothold, and the sacrifice might save a later headache.

As always, stay on top of your maintenance schedule. Conditions that allow unwanted microorganisms to thrive will lower your chances of success dramatically. Many chemotherapeutants used either prophylactically or as treatment will readily react with organics present in the aquarium - an excessive presence of these organics may lower your effective dose significantly.

Prophylactic treatments can either be administered as a short-term, once-off bath treatment, or a prolonged immersion where the chemical is maintained at a low dose throughout the incubation period. The bath method may be repeated several times a week, but usually this is not necessary unless you’re regularly encountering egg loss and resultant fungus.  Many of the chemicals utilized as prophylactics can be used once the problem has begun to occur, but it is always best to get ahead of the issue. 

Chemical cures

Many chemical treatments do not got well with filters. Carbon filtration will take up the chemicals, while the treatment itself may negatively impact your biological filter by harming the resident bacteria. It is better to treat the eggs in a separate enclosure to avoid these hassles. In days past, malachite green and formalin saw more abundant use in aquaria. Although some preparations still use these ingredients, they’re less prevalent owing to restrictions in several regions, and may yet disappear from the market altogether owing to some of their more dangerous properties.

While malachite green retreats from aquarium use, another dye, methylene blue, sees perhaps the most widespread use with fish eggs. It’s quite safe to dose over a longer term, usually during the entire incubation process, and is available under a number of trade names (or included as one of the ingredients of many others).

Many old hands in the hobby have a keen eye for the shade of blue they like to turn their water; for the rest of us, it’s usually recommended that you dose the incubating eggs at around 3ppm (3mg per litre). This concentration will usually be enough to eliminate currently present water mould, and the dosage should dilute over time (through break-down and water changes) to the point the water is barely tinged by the stage the eggs hatch.Dyes have strong staining capabilities, including but not limited to your clothing, hands, and aquarium silicone. Work carefully with these chemicals.

Hydrogen peroxide is a cheap and readily available chemical from pharmacies, found in a number of dilutions; for our purposes, the 35% mix is around where we want to be. This is one of the harsher concentrations, and while it will dilute in water do be careful when handling it.

Recommended doses vary, although I prefer the one that plays it a little safer — about 1ml per ~3.5 litres, on the first day, repeated again on day two. This second dose is useful since hydrogen peroxide breaks down in the presence of light as well as readily reacting with organics, of which there are plenty in the aquarium environment.

Salt has shown to be efficacious against water moulds at doses as low as 3ppt, and depending on circumstances can be raised higher than this. How useful this treatment may be is reflected by the salt tolerance of a given species. Not all fish, and by extension their eggs, will appreciate even slightly raised salinity. Indeed evidence seems to indicate this method works more favourably with fishes adapted to harder, alkaline waters (and, of course, brackish-water fishes).

Be sure to do any sort of salinity adjustments (both increasing and then later decreasing) very slowly.  There are other chemotherapeutics options available, inclusive of several commercial preparations that may use chlorine oxides, acriflavines, bronopol, or a combination of these (or other) active ingredients. Follow the manufacturers’ instructions closely for best results, but pay close attention to success rates of your batches. Tolerances to these chemotherapeutants can vary from species to species, with some being more sensitive than others. Experience here is handy in making the necessary adjustments.

(Ed’s note: Pimafix, with the natural phytochemicals of Bay oil, is also an increasingly modern method of Saprolegnia control.)