An article by Alysonpeaches
If your livebearer is female and has been in contact with males of the same species, the answer is undoubtedly yes.
Male livebearers have a modified anal fin called a gonapodium which they use to transfer packets of sperm to the female. The gonapodium is a rod like shape which can be rotated at any angle to fertilise the female. The female has a triangular anal fin, and is generally plumper than the male. Using these differences, even my 5 year old can tell them apart!
Guppies and some platys have gravid spots at the bottom of their abdomen. This is a dark spot that generally means fry are present. The black spot is the dark eyes of the babies which show through the skin of the female. Mating for many livebearers is a very fleeting experience, and usually consists of the male thrusting himself at the female and fertilising her with the gonapodium.
You will have seen male livebearers constantly chasing females up and down the tank. For this reason it’s a good idea to have more than one female to each male, so no individual female gets too harassed. Some sailfin mollies do a bit of a courtship dance, but they are generally the exception (other more unusual livebearers, e.g. Jenynsia multitdentata are beyond scope of this article).
Once fertilisation occurs, these are the average gestation periods:
Guppy 21-40 days
Platy 30-40 days
Mollie 4-6 weeks
When the female is ready to give birth she will be very fat and her abdomen is likely to have a "squared off" or boxed appearance, i.e. It is almost cuboid. The dark spot will be very evident. She will very likely find somewhere quiet to hide, she may go to the back of the tank or hide in a plant.
When birth is imminent, some people confine their livebearers to a breeding trap or a separate tank. This will undoubtedly mean that you catch more fry, but it may stress the female. It depends what your motivation is with growing on the fry. The fry are born live and often flee to safety of plants. For this reason, if you have a tank with java moss, cambomba or floating plants, you will save quite a few fry without having to stress out the mother by catching her and confining her to a trap. If you don’t want your tank overrun with hundreds of fish, they it’s perhaps best to let nature take its course. Let the mother birth in a community tank, some babies will get eaten, but often the strongest survive. You might get just one or two surviving from each drop, but at least you won’t have an overpopulated tank. Some mother livebearing fish are not averse to eating their own young, so move them out of the breeding trap or tank if you don't want this to occur. An average livebearer can drop 20-40 fry, but some manage as many as 150. (Baensch 1982)
As livebearers reproduce so rapidly, if you don't want babies, keep the males and females separate. Or right from the start, just buy all one sex, I kept a little shoal of all male endlers for quite some time and they were happy enough. Reproduction is still possible once the sexes are separate, as the female are said to store sperm from previous matings, but this would generally be used up after a while. Livebearer fry often mature early, male guppies are sexually mature at 3 months, so as soon as you can tell sex differences, you may want to separate them. A magnifying glass is sometimes necessary and it’s very very time consuming.
If you particularly want to raise a lot of your fry, a growing on tank is a good idea. You can feed them frequently and control the water quality. Some fry need feeding about 6 times a day, but this isn’t practical in a community tank, you will soon have poor water conditions. However, in a community tank, the fry seem to be able to feed on micro-organisms present around plants and bogwood as well as crumbs of flake.
I use flake food crushed with a pestle and mortar for fry, but you can probably pulverise some flake food between two spoons. Some people feed hardboiled egg yolk to build them up, but it really fouls up your water. You can also buy specialised fry food, hatch brine shrimp or cultivate your own worms. If you have some fry in java moss in a community tank, it is likely they will find enough to eat, so don't do what I did first time I had platy fry and deliberately put a quantity of crushed flake into the moss. This just attracts the attention of bigger fish and they may eat the food and the fry too!
If you keep your fry in the community tank and they keep breeding, please take time to review your stocking levels relative to your tank size. It would be extremely unfair on parents and fry to have an overstocked tank. Sometimes, local fish shops might take a few off your hands, or you might be able to sell or swap them if you have a local aquarist society.
Riehl and Baensch (1982/1986/1991) Baensch Aquarium Atlas, Mergus Germany
Lambert (2001) A Practical guide to Breeding your Freshwater Fish, Barrons New York
Hieronimus (2007) Guppies, Mollies and Platys, Barrons, Germany