There is no way of telling how long it will be before your molly gives birth unless you know when she last gave birth. They carry the eggs and developing fry (baby fish) for about 1 month. If she only has a small batch (5-10 fry) in her, she could give birth tomorrow or next week. If she has a big batch (100+ fry), it could be in 3-4 weeks.
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White poop is either intestinal worms or an internal bacterial or protozoan infection.
If the fish is eating normally and doing white poop it is usually intestinal worms. Praziquantel can be used to treat tapeworm and gill flukes in fish. Levamisole is used to treat thread/ round worms in fish.
You treat the fish once a week for 3-4 weeks, and do a 75% water change 24-48 hours after treatment. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Increase aeration/ surface turbulence and remove carbon from the filter when treating with any medication.
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If the fish stopped eating, bloated up (got fat) over night, and is doing a stringy white poop then it has an internal bacterial infection and will probably die.
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If the fish is not eating normally but still picking at food, and losing weight over a week, then it probably has an internal protozoan infection.
Intestinal worms are the most common cause of white poop in livebearers (mollies, guppies, platies, swordtails).