Hi and welcome to the forum
Do not add any new fish for at least 1 month. You should also keep the feeding down to once every few days and do a 75% water change and gravel clean the substrate 4-8 hours after feeding. Do this until you know the filter has established.
If you post a picture of the plants we can hopefully identify them for you.
Is there a light on the tank?
If yes, how long is it on for each day?
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You should check the general hardness (GH) and pH of your water. Mollies need water with a GH above 250ppm and a pH above 7.0. If the water is too soft (lacking minerals) or the pH is below 7.0, the mollies will have issues. At the other end of the scale, cardinal tetras and angelfish don't do as well if they are kept in hard water with a high pH. Normally I would not recommend keeping these fishes together because they require different water chemistry. However, it will depend on your actual water.
You can usually find the GH & pH on your water supply company's website or by telephoning them. When you check the GH, see if it is in ppm or dGH, they have different meanings.
If you can't find the information, take a glass full of tank water to the local pet shop and ask them to test the ammonia, nitrite, nitrate & pH. Take a separate sample of tap water to the pet shop and get them to check the nitrate, GH & pH of the tap water. Write the results down in numbers and post the results here. If the pet shop says "your water is fine", ask them what the results are in numbers.
If you want to get your own test kits to test the water, buy a test kit for ammonia, nitrite, nitrate & pH. GH does not normally change rapidly and you can get the local pet shop to test that a couple of times a year. The other tests should be done regularly or whenever the fish are looking unwell.
If you buy test kits, check the expiry date and don't get kits from warm areas like a fish room, next to a window or near a heat source. Heat destroys the chemicals.
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To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
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You don't need to do anything for the female molly except make sure the water is good and there are lots of plants in the tank. Water Sprite (Ceratopteris thalictroides/ cornuta) is the best floating plant for livebearers (mollies, guppies, swordtails & platies). If you get too much you can plant it in the gravel where it grows into a nice light green shrub.
