He's on drugs! Read your package, it says it will make 128 gallons that contain 800 mg. If your chicks drink that water it could kill them. The color should be a very light yellow, not brown.
It's not bad, I just wanted you to see the math.
Your vet over charged you, by a lot, but your vet screwed up by not telling you how much to use. Shame on him!
This package weighs 280 grams
Package contains 102.4 grams oxytetracycline
102.4 / 280 = 0.37 , which means that it's 37% oxytetracycline and 63% filler
370 mg per gram
One teaspoon probably weighs 3 grams, so it's 3 x 370mg = 1110 mg per teaspoon
The 800 mg dose for one gallon = ~3/4 teaspoons
Okay, that might help. If it doesn't you might want to try Baytril. Baytril is banned for use in poultry, but it's a much better drug for treating respiratory infection than tetracycline.