Chicken flock has respiratory illenss --- unable to give Tylan injections

My flock of 18 adult laying hens (& 1 rooster) has a respiratory illness. I had given them Tylan 50 orally (via syringes) for 3 nights straight ---- a few times. Last time was at least three weeks ago. They improve even with the three days' worth. They have had this problem for a few months --- hoarse coughing during the day. No other symptoms. My main problem is that I have temporary health issues myself, and don't have help with this endeavor. I'm unable to stay up at night to treat chickens right now for the full five days recommended, though I will hopefully be getting better within a few months.

I still have a good supply of the Tylan 50 injection (not powder). I had read somewhere that you could add it to the drinking water. I have a couple of nipple bucket waterers. Would this be a good second alternative, and if so, what dosage do you recommend?

Any other suggestions? I need for them to get well. They are still eating fine, laying and walking around, etc. Just don't sound good.

Thanks!