My Molly is Shaking

Question
Hi,
  I have a 50 gallon tank with 3 molly's, one starter goldfish, two crabs, a suckerfish and an albino dwarf frog. They all live happily with three homes and many plants. My silver molly, Polly, just gave birth to about 20 fry around a week ago. They are big and healthy, and everything is going good until today. I was looking in my tank, and noticed Polly was shaking. It looked like she was shivering. I don't know what can be wrong... she swims around, through every plant, in every house, eats normally and LOVES my starter fish, Tim. I can't imagine what can be wrong. I have been cycling my tank for about two months and just got her along with another molly last week. I knew she was pregnant and put her in a birthing bin. Her babies are in there still (so they don't get eaten) but Polly is roaming around. I put some aquarium salt in and check the PH levels and ammonia and all the hardness and everything twice a week. Am I doing anything wrong? She doesn't have anything on her like ulcers or cloudy eyes. When she is at a certain angle you can see a huge black blob in her like when she was pregnant. Can she be pregnant again? Thank you for helping me. It means a lot!

         Thank you,
         Megan

Answer
Megan,
Mollies are brackish fish and need salt in their water, so adding it will help. They are also tropical fish, where the goldfish is a cold water fish and they should not be in the same tank. Live bearers are able to hold on to sperm and able to have babies every 30 days. As for the shaking, it is called shimmying and it is usually caused by stress. I would remove her and place her in your hospital tank and keep her quiet, with no light foe a few days and she should be fine. Make sure the ammonia and nitrite levels are zero.