By Donna M. Recktenwalt
There are probably almost as many ways to incubate plant-spawning killifish eggs as are there are killifish keepers. Relevant topics for discussion always seem to include the best methods for collecting eggs, for incubating and hatching them, and for raising the resulting fry.
For fish that spawn in plants, the “natural” way is simplest. Either remove the parents to another tank after a week or two, or let the eggs hatch and fry develop in the same tank with their parents.
For killifish that have spawned in mops, egg collection is straightforward: shake the breeding mops gently to flush out any fish hiding within, remove the mops from the water and squeeze out, then search for eggs by pulling aside two or three strands at a time in bright light. The eggs may be removed from the mop for hatching in a separate container either in water or on peat, or the entire mop may be removed for hatching elsewhere, without handling the eggs at all.
One thing that may affect hatching success, and one that few people think much about, is cleanliness of their hands. Did you wash your hands before working with the fish or their eggs? Have you recently worked with materials that could have an adverse effect? Even slight chemical residues can have immediate (and sometimes fatal) consequences in aquaria.
For species that produce eggs more than usually sensitive to handling, the handling itself can be disastrous. In such cases, the entire mop can be removed to a separate hatching container or a plastic storage bag. This removal technique has proven useful for such fish as Epiplatys annulatus and some of the rainbowfishes, but can be used for more common species as well.
Whatever incubation technique you choose, fish eggs must have water to develop properly. But what water should you use? Many killikeepers use water from the parents’ tank. Some suggest using aged new water. Still others swear by distilled or reverse osmosis water. Some insist that a medicated hatch water mix is crucial to success.
Can the eggs be exposed to light, or should they be kept in darkness? Adherents from both points of view point to success in hatching eggs and raising fry.
Should You Medicate Hatch Water? “I would see if the eggs develop without using any antifungal chemical,” says Ralph Taylor. Fertilized, viable eggs seem to develop no matter the conditions, surviving even fungus and neglect to produce viable fry. Aquarists have also noticed that when fish are just starting to produce eggs fertility (viable eggs) is often very low. As the fish mature, fertility improves.
For those concerned about fungus and bacterial contamination of the eggs, several hatch mix formulae have proven useful.
Methylene blue is a biological dye which a number of bacteria can use as an alternative to oxygen for respiration; when used this way, the dye turns colorless. This “favoring the bacteria” approach may put fungus at a disadvantage, thus providing some additional edge to egg development.
“I put a small amount of methylene blue in with new eggs,” says Donald Nute. “This will not keep an infertile egg from fungusing, but it will slow down the poisoning of fertile eggs by the toxins the fungus produces.” When eggs start to show development, he moves them into clear water.
“I see little hatch-rate difference from any dyes I add to the water,” says Wright Huntley. His hatch mix? To one gallon of reverse osmosis water add a teaspoon of salt, 5 drops methylene blue, and fungus guard or Binox (made by Jungle Labs).
Donald Nute stores his incubating eggs in the drawers of a small plastic storage cabinet. The eggs are stored in methylene blue treated water and are checked daily, with fungused eggs removed. Developing eggs are moved into clear water which is changed once or twice a week.
Acriflavine. “I used to use methylene blue as an antifungal agent for all of my eggs,” says Cal Him. “Then I switched to Acriflavine. I put one teaspoon salt, 4 drops Aquarisol and 1 drop Acriflavine into one gallon of water.” Acriflavine Plus is an entirely different formula and contains Malachite Green, which may harmful.Incubation and Hatching
Hatching containers may be any small, lidded, easily handled glass or plastic containers that can be readily cleaned and which will keep eggs from clumping together, thus fostering direct cross-contamination. Small baby food jars, petrie dishes, margarine tubs, even the small sauce or dressing containers from restaurants have been successfully used for incubating eggs.
Some aquarists have their best hatches by using fresh aged water and changing it every day (or as often as possible). Water may be gently poured out and replaced in the original container, or the eggs may be poured through a fine mesh net and then moved into a clean container. A sprig of java moss may help keep the water cleaner.
“Clean [water] and high oxygen content seem to be the best qualities for water incubation,” says Wright Huntley, “but doing it on damp peat is even better.” Incubating the eggs on damp peat (just slightly squeezed out, so the surface is still wet but not liquid) lets you stall hatching for a few days. If you are collecting just a few eggs a day, you may be able to hatch a weeks’ spawn at once, making rearing easier. Be certain to check for bad eggs every few days, and for eye development at 3-4 weeks. Most eggs will hatch on their own when they are fully developed. Eggs that are ready to hatch, both peat- and water-incubated, will show a clear eye-ring. Eggs can be hatched out in a shoebox or other suitable container, using water from a tank with plants. After a day or two, add some snails and some duckweed or java moss.
But sometimes eggs don’t hatch. What then?
When the eggs look well developed, “I put them in a 35mm plastic film canister and carry them in my pocket” for a few hours, says Donald Nute. “Then I can pour the fry out into a container with aged water and a bit of java moss.” This procedure results in about 90% of the eggs hatching at one time. Left alone, the same eggs would probably hatch out over a period of several days.
A few alternative methods to force hatching:
- Put the eggs in a small, half filled bottle, then exhale into it (there is 90-100 times the amount of CO2 in exhaled air than there is in the atmosphere). This technique has even been successfully used to encourage “reluctant” annual eggs to hatch.
- Add a small measure of microworms to the hatching container.
- Bring down the pH slightly by adding a peat pellet to the hatching container.
- Whether it is the warmth or the decreased oxygen levels that work, try making a complete water change on the eggs using water that is 10°F warmer, or move the incubation container to a warmer location.
Sometimes, however, we just have to admit that those eggs won’t hatch.
Better luck next time!
— G.C.K.A. Newsletter – August, 1999