Raising young Medaka

A lot of Medaka keeping is very easy, they are quite hardy, but one aspect is a little harder then all the rest. The growing of extremely young fry. They’ll breed quite freely and you probably will get lots of eggs if you feed them well and give them temps above 18c.

Breeding them is easy enough, male and female, good water and fed well and temps above 18c and you’ll get eggs and they’ll hatch easily given a lack of predators. Funnily enough like most fish keeping, natural methods are the best. If you want an easy way to raise young fry, a nice outdoor pond with heavily planted environment and a rich biotype with green water and micro and macro fauna and the babies will grow, now this situation is good for the hands off breeders. You will get less fry surviving to adulthood but you also don’t actually have to inject any effort in raising them. Natural attrition of babies is a natural method of ensuring only the toughest and hardiest specimens survive, those able to evade the predation from parents, dragonfly nymphs, birds and other predators. You’ll have a handful of babies make it through from each batch but they’ll be very hardy.

The above method is not great for producing a lot of fry with the phenotypes and characteristics for a selective breeding program. You want the largest number of specimens so you can pick the best features and characteristics from the sample base. So I’m going to share the way I raise fry in hopes the information helps or makes any or all options of raising Medaka fry available to you.

I use breeding mops made from scrubbing pads with fingers on them in breeding tubs, the least amount of physcial objects in the tubs the more eggs you will be able to harvest. Females must rub eggs off onto objects, the natural instinct is to rub the eggs onto plant stalks or any object that will shield the eggs from predation, often inbetween small crevices in plant material. Fingered scrubbing pad mops are good, theyre very course and will attach eggs easily and also very easy to remove eggs from.

So a mop will be spawned on is removed from a tub, pond, whatever you have the adults in. I use 3-5litre containers to start with, at the start I fill with 50% greenwater and 50% mature water, I put small amounts of scoria in this container with a very slight bubbled airflow, just so the bubbles very gently break the surface tension. I also put them in an environment where it is 19-23c constantly, this makes sure there are no massive drops of low temp that would kill very young fry. This might just mean having small tubs indoors. I also put a few mature daphnia in there and then just leave it. I leave the mop in there for 1-3 weeks, periodically checking to see if all eggs have hatched. Water temp and genetics will plus or minus the amount of days eggs take to hatch. If there is only 1-2 eggs left I will normally pick them off onto a newer mop. I put the mops in newest to oldest and then check the oldest until all the eggs hatch. Mops with all removed eggs are put to dry for a week, next to my heater to cook off debris and snail eggs etc.

Eggs will hatch and the babies will be very small and constantly on the surface. At the start its very hard to raise them, cause the food that they can eat is hard to get into them. Normally the daphnia will have eggs and newly hatched daphnia babies in the tub, also if the tub has been running for awhile it will have mulm and a decent bacteria colony inside for very young babies. Now the amount of food in the tub is not genuinely regenerative, but it has the start of a complex ecosystem, when they are young its somewhat fine. I dont just assume its good, I keep adding and testing, the food is added even if no hatched young or present, cause it also feeds the snails and daphnia. Everyday each 3-5L tub will add a 1-2 snuff teaspoon of 100-200micron powered food into each tub, which will feed both larger fry and also feed daphnia/snails and a 20-50ml squirt of greenwater from my greenwater supply and 20-50ml squirt from a turkey baster from my infurosia tank. Now my infurosia and daphnia supply probably has a complex other fauna mixed in like Copepods/Ostracods etc, nothing I really intend but are probably in there. The smaller tub with large surface space, means all food in the container is easy for small fry to find. This mix of food ensures at least something is being eaten by them or feeding the thing that is feeding the baby medaka.

I believe in keeping this feeding simple, I dont measure anything indepth, the snuff spoonfuls are based on how many fry I think are in there, the squirts from the plastic squeeze wash bottle with greenwater and turkey baster/infurosia are just approximates on what I think they need. I try and keep it simple so all up the while process of feeding takes about 5-10 minutes, even with 30-40 tubs. Something I can easily do multiple times a day.

I feed this mix of powder/greenwater/infurosia until they reach a certain size (2mm) or the tub gets too over populated then I move the whole 3-5L and dump it into a 20-25L container, mostly because the larger fry harass the smaller ones as they hatch and as they get larger they need a larger container and most importantly they can find food easily in a larger container. Above 2mm you can probably move to an exclusively powdered diet, I don’t but they probably could easily do well on this only. Above 2mm the fry are fairly easy to raise, and there is not much to it. Past this point its simply waiting for them to grow and put on weight and it becomes vastly easier to raise them at this stage.

I hope this info helps in deciding the best method of raising very young fry.