You may need to give more info then "Lyretail". First guess, it is a molly or platy, which are both livebearers so no eggs. If you don't have room for more fish or tanks and don't really care to keep offspring, then just let it happen. You could add alot of plants for cover to let a few survive, if the fish doesn't abort. With cover, in a few months your tank could be overloaded, so keep that in mind. If you are not sure of the stage the fish is, then do frequent PWC's, this usually helps induce labor. Do the PWCs until you see the fishes belly go back to normal, then with in a few days you should be able to see if any offspring survived.

I would buy a "Guppy Trap", which would isolate the mother and provide a slit for the babies when they are born.

hi
I am planning on purchasing a 20gal. tank and i was wondering if I could put a rainbow shark in it. If so what fosh and plants would best suit the shark?

Hygrophylla, Water Sprite, Bacopa, Vallisneria, Cabomba, to name a few.