First off, welcome to the forum.
Mollies are pretty big fish and don't do well in breeding nets. Nets can stress them out, especially when they are kept in there while waiting for birth. Stress can lead to less immunity to disease, birthing difficulties, abortion of fry, and even sometimes the death of the mother. I'd suggest you don't leave them in there for more then 2-3 days, and personally, I'd take them out now. If you add in some plants (floating ones especially) and let the mothers give birth in the tank, you'll get some fry most likely. It's also less stressful on the mother.
Fish usually give birth overnight, but there's no set time. Anytime you're not there seems to be the rule of thumb.
Signs of labor may include: increased or decreased appetite, hiding, chasing fish away from her hiding spot, overall aggression, hoovering in one space, breathing harder or lying on the gravel. Not all fish show all of those signs. Some fish don't show any at all. You also have to be careful because a few of those signs can also be symptoms of diseases. Which brings me to my next point. When you say your Molly sinks to the bottom, that doesn't sound too good. Is she able to swim otherwise? What's your water quality like?
One more point from me: Mollies have a few requirements in terms of water. They prefer a bit of added salt into the water, or if that's not possible, hard and alkaline water. If you have soft, acidic water, you'll want to add salt. Without these conditions, they are more prone to diseases, especially "shimmies". Not all fish can tolerate salt though, so be careful who you keep them with if you are going to add it in.
And last but not least - I noticed you keep Goldfish. I'm assuming they are in a different tank then the tropicals, since they are coldwater fish?
Anyway, good luck with your Mollies and keep us updated!
