Spinner?

I found a newtpole in my small larva tank that I thought was dead as it was near the surface. When I went to fish him out, he moved. He can swim straight, it seems, but seems unsteady - sometimes more vertical in the water, sometimes spinning. I read the article on Why Larva Die again, and I'm not sure what to do. I immediately tested the water and all is well w/ ammonia and nitrite - didn't test nitrate, but can if you think it's a problem. I have a bit of visible food in the bottom of the tank - results of frozen Daphnia-like blend and some live Daphnia. I change at least part of the water daily. It is in a small tank w/ 10 siblings - say ~ 2 Gal. filled a couple of inches w/ a few fake plants only. I separated the larve - not sure I can/should euthanize it or not. I definitely never noticed this swimming behavior before and it's not a newly hatched larva - has to be ~ 2 weeks old. The four that are 4+weeks old have been moved into a separate tank.

Why is letting a spinner or weaker larvae live w/ the others jeopardizing the whole bunch? Is there hope for this one who just developed the odd swimming patterns?

Dana