There isn’t actually a specific age when you have to jar fry. The growth rate of the spawn can vary widely depending on genetics, water changes, food, etc. The general rule is 3 months of age but it is not a hard and fast rule.
When do the males need to come out?
I jar them anywhere from 10 weeks to 3 months on average. I can tell they are ready when the males start to be identifiable by longer fins, stronger color and ATTITUDE! They will flare at other fry and start to tear each others fins. There is not really a point in jarring all the fry at once. Doing this means you will have to change more jars, sooner! Start by taking out the most colorful / aggressive males. As you take out some, others will take their place and become dominant/colorful/longer finned. Usually by 4 months of age, all the males are out of the aquarium.
Should you jar the females?
This is completely personal choice. If you are not going to show your fish and have the space to let the girls grow out in a large enough aquarium, they can stay in the grow out aquarium until you need the space. Eventually I do jar the females just because my space is limited and I will need the grow out aquarium for the next spawn. You can however in theory, leave them together indefinitely as long as they do not become overcrowded. Do make sure to take out any super aggressive females as some do attack the other females with more vigor. A few fin nips will also be normal so if you do not want torn fins, you may also want to jar them.
Consider the number of fry you are raising…
If you have so many fry that you do not have space to jar them or time to clean that many jars, consider finding some of the fry homes at a young age before jarring. Thinning out a spawn early saves time, space and generally cuts down on grow out time since you are likely to offer the smaller fish. As a warning, the smaller fish to grow up to be big fish (usually) so you could be tossing your best fry if you give away or sell them young. I generally take this risk on very large spawns because my time is limited. So you will often see at least a few fry offered from larger spawns. Smaller spawns I generally keep until the fry are larger (100s of fry vs under 100….).
What to put the fry in…
Well cleaned (don’t use soap) plastic or glass containers. Ideally quart or half gallon (or larger if you have the space) work best. Adding a small bunch of plants keeps the fish happier and less likely to jump out. I like java moss for this purpose and it also improves water quality. I lack the time to have a lid on every container (I want to be able to feed all the fish in reasonable amount of time in the mornings) so I lay plastic craft canvas over my jars/containers. Most of the time this keeps the fish in but not always. Other options are to use lids but drill/poke a hole in them for feeding. In the past I have melted holes in beanie baby cases with a soldering iron both for air exchange and so that I could feed the fish easily. If you have irreplaceable fish, a lid is recommended. “Kritter” keeper containers also work well since the lids are ventilated well and keep the fish in water. My containers of choice are quart deli containers, CCW 3/4 gallon plastic jars and half gallon rectangular beanie baby cases. Glass canning jars also work and I have used those in the past when the other containers were unavailable.
To see or not to see…
Let newly jarred fry see each other! You have taken them away from their siblings and placed them in a container where they cannot swim freely. This is scary to them. It makes it easier on them if they can see their brothers or sisters for at least a week or two. After that, if you plan to show your fish you may want to “train” them by carding them with paper or cards so that they cannot see each other. Then remove the card 20 minutes before feeding, then put it back. This gets them to flare on command and it is also useful when getting fish ready to breed. Solitary confinement encourages breeding behavior in both males and females so it isn’t a bad idea to card your fish about 2 weeks before you try to breed them. It makes them much more interested in each other when they are placed in a spawning aquarium.