Artificial Hatching of
Bacteria-Challenged Fish Eggs
By David Snyder
Nov-Dec 1998 Issue# 18
All fish eggs look the same. Well, maybe not all fish eggs. But most eggs of substratum-spawning cichlids look pretty much alike. So it's remarkable that some are extremely resistant to fungus and bacteria, and others will turn white and cloudy no matter what you do.
The common Severum (Heros severus) is a fish that lays lots of eggs (up to 1,000 per batch), and once they get going, they will spawn twice a month. But their eggs are very susceptible to bacterial contamination, and so it can be a challenge to achieve a decent hatch rate. This article describes a method I have used successfully to hatch Severum eggs. I believe it could be applied to most substratum-spawning cichlids whose eggs turn bad easily.
First I'll describe what doesn't work. Since Severum eggs look a lot like Angelfish eggs, I figured that they would hatch with the same anti-fungus treatment. So for my first batch I removed the eggs from the parents, placed them in a 2.5 gallon tank, and added enough methylene blue to turn the water dark blue. I placed an airstone in the tank so that the bubbles were near the eggs, and waited.
This tried-and-true method produced a hatch rate no greater than 10% at best, and 0% at worst. Now, 10% of 500 or so eggs is 50 fry, and once they do hatch, Severum fry are very hardy. Raising a batch of almost 50 Severums is nothing to sneer at. Still, losing 90% of the eggs feels like a failure, and I was determined to do better.
So I figured perhaps it would be better to let the parents hatch the eggs themselves. Well, although my pair are excellent parents, this was disastrous, as batch after batch went completely bad, no matter how many water changes I made or didn't make, no matter how much or how little I fed the parents, etc.
So I went back to artificial techniques. Next I tried the same method I had used at first, but I replaced the methylene blue with acriflavine, the anti-bacterial chemical of choice for Killifish eggs. The result was even worse than the methylene blue method -- a 0% hatch rate every time.
For my fourth experiment I went back to the methylene blue, but this time with a vengeance. Besides being ultra-sensitive to bacterial infection, Severum eggs take three days to hatch at 80 degrees, compared to the two days in which Angel eggs hatch. Now, this is definitely a problem, because although methylene blue is an effective anti-bacterial agent for 24 hours or so, it oxidizes quickly and loses its effectiveness. In short, the longer the eggs take to hatch, the greater the risk that they will be unprotected.
I noticed that most of the Severum eggs went bad after the first day, and in fact almost three-quarters of them lasted until the end of the second day, only to go bad just hours before they should have hatched. So this time I added a new dose of methylene blue during the second and third days, so by the time the eggs hatched, the water was so dark it was almost black.
This ultra-dosing with methylene blue worked, at least it worked better than anything I had tried so far. I got about a 25% hatch rate, although by the third day the water was so dark it was difficult to see what was going on in there. But at least I was on the right track.
So the key is to dose with methylene blue every day, I thought. But the method could be refined even more. For one thing, a 25% hatch rate is still no great accomplishment. And for another, to get rid of all that dark blue water I was changing water three times a day once the eggs hatched so that when the fry became free-swimming, they could see food and I could see them, and that's just too much work.
Now it occurred to me that since the methylene blue loses its effectiveness after 24 hours anyway, why not just replace it every day, rather than simply adding more. So I finally hit upon the method that has given me my best success rate with Severums to date, a 75% hatch rate.
As before, I set up a 2.5 gallon hatching tank and put the eggs into it. But this time I used freshly-drawn tap water to fill the tank instead of water from the parents' breeding tank. This gave the eggs a head start, because chlorinated tap water is designed to kill bacteria. I discovered that the eggs seemed to be completely unaffected by the amounts of chlorine in Grand Rapids city tap water, even though straight tap water might kill fish. I placed the egg-containing receptacle in the tank so that the eggs were facing downward. This is important so that detritus-containing bacteria can't settle on the eggs. As before, I situated the airstone so that the bubbles came as close to the eggs as possible without touching them, and I turned up the air flow until it was pretty vigorous.
Instead of adding just methylene blue this time, I also added a mixture of acriflavine and malachite green sold under the trade name of "Acriflavine Plus." I have used this product in the past with methylene blue to hatch Angel eggs, so I figured it wouldn't hurt with Severum eggs either.
So far I did two new things: (1) I used straight tap water, and (2) I used a mixture of methylene blue and Acriflavine Plus. While these changes probably helped, the real key to a better hatch rate was yet to come. Remember, methylene blue loses effectiveness after 24 hours.
So this time I siphoned all water out of the hatching tank every 24 hours until the eggs hatched, replacing it each time with freshly-drawn tap water of about the same temperature. I also added a completely new dose of methylene blue and Acriflavine Plus. This solved the problem of the water turning so dark that I couldn't see anything in the tank, with the added bonus of infusing a new dose of chlorine each time I changed the water. So by changing the water each day I got rid of most of the bacteria that managed to survive the previous day's chemical barrage, and I subjected them to fresh chemicals.
The result was a pretty good hatch rate of about 75%. Now, I still haven't achieved the 95%+ hatch rate I normally get with Angel eggs, but I'm not sure what I would do with 500 Severum fry anyway. Raising 375 or so Severum fry will just have to do. The End