If we gently herd a female guppy into a glass jar (soap-less of course) we can then shine a flashlight from behind her. We should see eggs and/or developing fry, if the female guppy has much size on her.

If we can't see eggs, she may just be full of the tissue, which sometimes finds its way into older fishes. More disturbingly, she may be afflicted by parasites, especially parasitic worms such as Camallanus or Capillaria. Please read the Camallanus section of Immediate Help.

If you suspect that your female guppy (or any other fish) is infected with internal helminthes (worms) either treat the aquarium with an anthelmintic (lots on these in that section on Camallanus or Google Guppylog for anthelmintics). If you use one of the medicinal mixes please consider using a hospital tank or bowl. Those treatments from Jungle and others usually include both anthelmintics and antibiotics. The antibiotics will destroy the nitrogen cycle in your tank.

You do need to keep that hospital aquarium warm, so will need a heater (I would suggest a submersible one) on your hospital tank. If you are as thrifty (cheap) as I am, you might put the said fish in a wide-mouth (soap-less) gallon jar. Previously add some of its aquarium water and set it on the strong glass top of a heated aquarium. The jar will stay close to the aquarium's temperature. Scale the medication to one gallon and loosely cover the top of the jar.

Of course if you have a second aquarium, which seems uninfected, you might wish to treat the original aquarium and then tear it down after the treatment period. Import 1/2 the water, an active sponge filter you had in the second tank. Also plants and gravel from that cycled tank can be used to help kick start the cycle in the treated aquarium.

These worms can go for a couple of months before we detect their presence. So a mere quarantine of 2-4 weeks will not get them - unless you do a routine anti-parasite treatment a couple of days before moving the quarantined fish to your regular tank(s).

There is a danger that that nets can carry almost microscopic worm larvae from one tank to another.    Buckets and siphons (and maybe out hands) can also carry them around, even if we do not see them. That is especially likely if Camallanus are extending from the fish's vent. (Again see the IM section.) In this weather in my part of the Northern Hemisphere, it doesn't hurt to leave that equipment out in the freezing weather when you are done with it. Scrub down you hands and keep your fingers out of your mouth. ;)