terramycin can upset their stomachs and cause poor or incomplete digestion as it upssets the natural balance of flora in the gut. If the chick's flora in the gut is disrupted, it will not absorb nutrients as well, which will EXACERBATE a growth problem, not help it.
I'd definately discontinue the terramycin. It's an antibiotic. Just being small in and of itself does not indicate any bacterial infection, generally speaking.
What I'd do is one of two options:
IF the chick is being picked on, then you may want to separate it or maybe put in a separtate group with just one other calm/less dominate chick.
If it's not picked on, then I'd just take the chick out 3-4 times per day and offer a separate meal of a mash made up of chick starter and warm (not cool, not too hot) water.
Being small can be a vicious cycle. Smaller chicks can't compete at the feeder as well as the larger and stonger ones. That restricts access to the feed, which in turn can contribute to failing to thrive/smaller chick, which just makes it harder for that chick to compete over time.
Personally, I don't use extra vitamins or electrolytes in the water, but others do recommend that as well - I'll leave it to someone more experienced and familiar with those products to make a specific recommendation on that.
Other information that might help are:
what breed are these chicks?
How old?
Are there any other symptoms other than just being small and not growing well? IE, any nasal or eye discharge, sneezing, lethargy or low energy/movement, any physical deformity like twisted or curled toes, etc...
I will post a picture of her when I learn how
