Couple Questions About Mollies and Platys

Question
Hi,                         

         My name is Chip and i'm 18 and i have a 20 gallon tank with i black mollie, a dwarf plattie, 6 three week old baby platties (in a breeding net), and a dalmation mollie.  I just had a few questions i was hoping you could answer if you didn't mind.

         First, when i bought the dalmation mollie about 5 days ago the lady at the pet store said she was pregnant and she would have the babies in a week or so i bought a breeding floatation device that allows her to have the babies in so she won't eat them.  But i'm not really sure if she is pregnant or not how can i tell?  She has small yellow dots around her stomach and its kind of big...

        Next, I was wondering when i could let my baby platties into the tank?  They are about 3 weeks old and the size of my finger nail.

        Last, I was wondering what are some tips on how to get nitrite out of the tanks system? It is at about 7 right now and i have been changing the water weekly and putting aquarium salt into the tank recently but it doesn't seem to be working too well..


                                         Thank You so Much for your time,
                          
                                                                            Chip

Answer
Hi,
Firstly, I would like to recommend this site:
tropicalfishcentre.co.uk
as I learnt everything I know now there, and everyone is very friendly and helpful.

It will be obvious if she is pregnant; she will grow very big. Don't put her in the breeding box, that often causes a lot of stress, but move the fry when they are born. Is the breeding box a box that floats on top of the water? If so, I would buy one that is a breeding net, instead of a box, and is just below the water surface. That means the tank water can flow in and out of the net, instead of just being one still box of water. Feed the fry crushed flake food, baby brine shrimp and liqui-fry.
You can let them into the main tank when they are about 2cm in length, to be safe.

I would doubt very much your nitrite is 7, more likely 0.7? 7.0 is just about impossible. Are you sure you're not looking at the nitrAte if it is 7? A nitrAte of 7 is perfectly acceptable.
I am not a fan of putting aquarium salt into freshwater, as it can easily go wrong and you could have a disaster on your hands, and it's not going to really help lower the nitrite anyway.
You need to do water changes every couple of days until it is 0.0, even 0.1 can be harmful. As long as yo do 30 percent water changes every day or two, it will go down.
Feel free to ask me any more questions you want.
Best of luck,
Kathryn