Yes, they loop A LOT, lol.
Do your best to keep their food and poop separate.
I'm not a vet and I've never tried raising them like you are, but I think about 1 tsp per chick for the tuna, try and find tuna that isn't super salty if possible. I have always fed my chicks free choice, as much as...
Hmm, you're out of the little area of knowledge I have on chicks, I've never heard of cold brooding and anything other than chick starter being used.
So just throwing out random things here...
Are you sure they are getting enough protein?
Do they have a warmer place to get to in case they...
To help kick things off, here are a few questions we'll need to know:
What are you feeding it?
What is the temperature in your brooder?
Have they been outside?
When you say you let them out, what do you mean?