Hi and welcome to the forum
Can you post a picture of the fish?
In the wild, female livebearers hang out in large groups consisting of between 20 & 50 (but sometimes up to 100 or more) females. These groups have a pecking order with the biggest most dominant female ruling the group and she has a group of girlfriends who back her up. All the other females live in the group but are lower down the pecking order.
The groups of females move around rivers and waterways looking for food and places to hang out. As the groups move around a few males follow the group and try to breed with any females.
In the confines of an aquarium, the males will constantly harass the females and try to breed with them. This puts undue stress on the females and if there are too many males constantly pestering the females, the females can get sick and die.
It is preferable to keep livebearers in single sex tanks (either male or female but not both sexes together). If you want a group of males and females then have 1 male and at least 6 females (preferably 10 or more females per male).
Female livebearers can carry up to 6 sperm packets from breeding with males and they use 1 sperm packet to fertilise each batch of eggs. The gestation period (from the time she fertilises the eggs to when she gives birth to free swimming babies) is about 1 month. After which she will fertilise another batch of eggs using another sperm packet. This allows female livebearers to produce young about once a month for up to 6 months without any males being present.
If you want to breed livebearers then have a tank with females and let them give birth and use up all the sperm packets they are carrying in their body. Give them a few months without being pregnant and then add a male to the tank for a week before moving him out, or move the female/s into a tank with a male for a week and let them breed. Then move the females back into their own tank.
-----------------------
It is preferable not to move pregnant fish because you can stress them and even damage the unborn fry. If you have to move pregnant females, carefully catch them in a net and then use a plastic container to scoop the female and net up in some tank water. Move her (in the net in the bucket of water) to a new tank and then carefully pour her into the new tank.
If you move the female to a smaller aquarium that would stress her.
If you use completely new water in the new tank it would stress her. If you set up a new tank for her you should fill the new tank with water from her current tank, so the water is exactly the same. Better still, move the male and leave the female where she is.
If the new tank does not have any plants in it will stress her. Put some plants in with her. Water Sprite (Ceratopteris thalictroides/ cornuta) is the best plant for livebearers. It normally grows on the surface but can also be grown in the substrate where it grows into a lovely light green shrub.
If you don't have any live plants then add a heap of artificial plants to provide her with shelter when she goes into labour, and to give the fry somewhere to hide when they are born.
Make sure the new tank has an established filter in it so the water stays clean.
Do not put the female into a tank with males until at least 1 week after she has given birth otherwise the male/s will stress her out.
Try not to move the female for at least 1 week after she has given birth so she can heal up. If you have to move the female after she has given birth, use the method above for moving pregnant females.
Most female guppies do not eat their young if the female is well fed and there are plants in the tank, but will eat them when they are confined to a small breeding trap.
-----------------------
If the fish have been really fat for more than 1 month, they could be full of worms. Livebearers like guppies, mollies, swordtails & platies are regularly infected with gill flukes and intestinal worms. If the fish are fat and eating well but haven't given birth in several month, they probably have worms.
You can use Praziquantel to treat tapeworm and gill flukes. And Levamisole to treat thread/ round worms.
Remove carbon from filters before treatment and increase aeration/ surface turbulence to maximise oxygen levels in the water.
You treat the fish once a week for 3-4 weeks. The first treatment will kill any worms in the fish. The second and third treatments kill any baby worms that hatch from eggs inside the fish's digestive tract.
You do a 75% water change and complete gravel clean 24-48 hours after treatment. Clean the filter 24 hours after treatment too.
Treat every fish tank in the house at the same time.
Do not use the 2 medications together. If you want to treat both medications in a short space of time, use Praziquantel on day one. Do a 75% water change and gravel clean the substrate on day 2 & 3. Treat the tank with Levamisole on day 4 and do a 75% water change and gravel clean on day 5, 6 & 7 and then start with Praziquantel again on day 8.
The water changes will remove most of the medication so you don't overdose the fish. The gravel cleaning will suck out any worms and eggs that have been expelled by the fish. Repeating the treatment for 3-4 doses at weekly intervals will kill any worms that hatch from eggs. At the end of the treatment you will have healthier fish and in a couple of months you should get babies.
