Yes, pet stores are famous for not knowing how to care for a turtle.
I have a compact fluorescent UVB bulb in addition to a heat lamp. Since you already have the halogen, I'd just get a UVB for now. It des not add heat and can be used with your halogen. I'm told the UVB output drops off before the visible light does and that the cfl UVB's need to be replaced every 6 months.
They do make mercury vapor lamps that provide both, but they are more pricey.
As to your desire to provide a more natural diet, rosy red minnows do contain thiaminase.
http://www.wetwebmedia.com/ca/volume_6/ ... minase.htmIn the wild, your turtle would probably be eating more insect larvae, small crayfish, freshwater clams/snails than fish. It certainly would not be eating just one type of protein. You may want to reconsider the pellets as a supplement with any live food.
I don't know that you need to move the tank. Our original tank started in my son's room. I started reading on his bed and Toby eventually started basking when I was there. Now the ank is in the basement and I never see him bask. I think you can leave the tank and your turtle will become used to the activity.