HI, MOLLYGIRL, WELCOME TO FISHLORE!
Your aquarium profile is confusing. How many mollies are in your tank? I counted 9. Is that right?
Are you testing your water? You said the tests are all 0....that couldn't be if this tank has been set up for awhile. Most here recommend the API Master test kit, as it gives the most accurate and easy to read result.
If I read the fish correctly that you have in your tank, it is severly over stocked. Couple that with only changing water once a month, and I can imagine you will start to experience many of the associated problems.
First get some good readings on your water tests. Next change water, 25-50% each week. Once you get your tests done, if the nitrate is over 20 you need to do 25-50% every other day until it goes back down to between 5 & 10...
My suggestion is to start looking for a bigger tank. You need to have that crowd in at least a 55 gallon....this would also give you the current tank for fry....Good Luck