Welcome to Tropical Fish Keeping forum. Sorry your first post has to be a problem, but we're all here to help.
I suspect that rather than one specific issue it may well have been a combination. The tank was not cycled when fish went in, new fish were added too soon, store water was added to the tank...any one or all of these are trouble.
First, cycling a new tank. I don't know where stores dream up this idea that running an tank for a week without fish somehow cycles it. It won't. There has to be a source of ammonia, usually from the first fish, and then bacteria have to appear (they do naturally but it takes time) and the cycle has to become established, which can take 2 to 8 weeks normally. Here's a good article on cycling from the "sticky" at the head of this section, have a read for background information on cycling:
A Beginner's Guide to the Freshwater Aquarium Cycle
I'm surprised the first molly in the tank lasted so long; mollies are highly susceptible to ammonia poisoning. Even if ammonia (and then nitrite) doesn't kill the fish outright, it severely weakens them internally and they may struggle on for weeks before dying.
Second issue was introducing new fish. Never add water from the bag to an aquarium. The bag water contains ammonia from the fish in the bag, and you don't need more of that; it may also contain pathogens, parasites, disease--who knows what may be living in the store tanks, even if the fish seem OK, they can still carry things with them. There are two similar methods of introducing new fish:
1. Open the bag, carefully remove some of the water if there is a lot, then float the bag in the aquarium for 20 minutes to equalize temperature; I do this by placing the bag at one corner and bending the top over the edge so the bag stays upright and the waters don't mix. After about 20 minutes add some of the aquarium water to the bag; I use a cup. Drape the bag over the frame again and leave for 15-20 minutes. Repeat with another cup of water. If the fish are particularly sensitive, I may do this 3 or 4 times. Then, with a small net, net the fish out of the bag and into the aquarium. Discard the bag of water.
2. Similar to above, float the bag to equalize the temp. carefully pour the fish and bag water into a small pail (only for aquarium use obviously), and run a drip line from the aquarium to slowly drip aquarium water into the bag. A piece of airline tubing works for this. When the water in the pail has doubled, which should take several minutes, net the fish out and into the aquarium.
Back to the present issue: as you have fry, I would leave the tank "as is" and let it cycle. Get yourself a test kit, API make a good one, liquid (not strips), a combo that includes ammonia, nitrite, nitrate and pH is sufficient. The growing fry (if they survive) will cycle the tank very slowly, but it will work.
Byron.
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