I have 3 week old adf tadpoles that are slowly dying off. I'm hoping someone can help me figure out what is wrong and how I can help the remaining live.

They were fine for the first 2 weeks and I have done nothing different. I have raised adf tadpoles perforce and the only thing I did different this time was fed microworms which were to smelly for me so I went back to just feeding bbs.

The tadpoles start by swimming awkwardly. Within a day it gets to the point where they spin instead of swimming. Then they can't even maintain their balance, hang out at the bottom on their side. Then by day 2 die

it follows the same pattern every time. And affect 1-3 at a time. I'm down to 6 left. All have ate and grown normally. The remaining have partially developed legs.