The largest spawn recording I have been able to find and study was over 700 fry. SEVEN HUNDRED. Them bettas sure can crank some eggs out.... Most tend to be smaller though, around a hundred, which is still nothing to shake a stick at.
Bettas don't lay all the eggs in one embrace normally, so if you are trying to limit your spawn size, then yes, some people do separate the male and female after the second embrace (not usually the first, because sometimes bettas don't always do anything in the first and you may end up with a spawn size of... zero). However, this is easier said than done, catching the female when they aren't done breeding without disturbing the male or his bubble nest is a difficult task. If you mess up the bubble nest, then there goes the whole breeding, as the eggs will fall to the bottom and not hatch. This is one of the many reasons why it is recomended to have a large breeding/spawning tank - a 20 gallon long is the norm from my research. This gives you considerably more room to be able to have the bubblenest at one end of the tank, and then be able to get the female towards the far end for ease of removal.
Amount of grow-out tanks you will need: Each male will need one. If you have a spawn of one hundred fry (which is not uncommon at all) and assume approximately half will be male, then you will need fifty jars. The larger the spawn, the more males you are likely to get, and the more jars you will need. Females can be left together in the spawn tank to form a sorority, but I've also read that it gets a bit crowded in there by about three-four months of age even after all the males have been pulled. Afterall, about fifty betta fish in a 20 gallon is way overstocked and you will quickly loose most of them to ammonia poisoning. Some breeders I've read like to keep a couple spare 10 gallons around, and they will separate a dozen or so females off at the same time into those to ease the crowding problems.
Fry in a one gallon tank: How young a fry are we talking here? I wouldn't jar a betta until they are over three months of age, at which point in time I don't think they are really considered 'fry' anymore. The process of netting or cupping a fry can be very taumatic to them when they are young (baby bettas break) and the fry may well not survive the transfer. Besides, its best to leave them in the larger tank until they get older so they can properly develop by being able to swim around the larger tank. And when you are putting a betta into a one gallon, we're going to assume it is a male over three months of age. You only stick one male in each jar, otherwise you're asking for fighting bettas. Don't jar multiple females in a single gallon jar. It's too small. Females get left in the sorority in the main tank, or if they are too aggressive and need to be jarred, its by the same rule as males, one to a jar.
Last edited by Nataku; 04-22-2009 at 06:50 AM.