The chickens will drink the right amount of the mix for their weight and age when it is mixed 1 tsp per gallon. Tylan is pretty good for some bacterial respiratory diseases such as mycoplasma (MG.) Denagard is used more in flocks with repeated bouts of MG to treat and to prevent recurrance. I would just see if the Tylan works. If it is just an outbreak of infectious bronchitis which spreads like wildfire through a flock, then it will have to run it's course over 3-4 weeks. IB usually just has the more mild signs of sneezing and runny nose, and maybe a decrease in laying temporarily. Antibiotics don't affect viruses, but may help to treat any secondary bacterial infections.