Dead Dalmatian molly and golden inca snail..HELP

[carrievs]

I am new to fish. I set up a heated fresh water 10g tank about a month ago. I have some grass and a money-wart plant. There are (were) two dalmatian mollies, one platy and a cat fish along with 3 golden inca snails. The dalmatian molly had babies last week (surprise) and we kept two of them and took the other 4 back to the fish store. I did have 3 guppies but they became aggressive after the babies were born so we took them back and got the platy. We had only had the fish for two weeks when the molly had her babes.

Everyone seemed to be really happy the snails have tripled in size i give them algae crackers. The fish seem happy and relaxed, i feed them fish flakes. I have changed 20-30% of the water twice in the past month. I came home today to find the smallest of the snails dead and then a few hours later the dalmatian molly (the one who had babies) was dead, i tested the water and the results are as follows:
GH - 60
kH - 80-120
ph - 7.0
NO2 - 0
NO3 - 40

Does anyone know what may have happened? i believe these levels are good. What can I do now? I now have one full grown molly, two 1 week old baby mollies, one platy and a cat fish can i get another fish? Can you recommend what kind?

 

[TMaier]

You need to invest in an ammonia test as well.
Did you do anything right before they died?? Water change? New decor?

 

[DBradar3]

Yes, you do need a test that shows ammonia, nitrites, and nitrates; ph is useful but not completely necessary. I have a liquid test myself that works very well and easier than people would assume.
That is actually a LOT of fish for a 10 gallon. Mollies are especially space users and need a minimum 20 gallon. You said you only had two mollies too. Were they both female or one of each sex?