Boy, do I feel for you.

I had 12 of 14 go through URI a month ago. Most of mine are semi-feral so medicating was almost impossible. I got some Orbax from my vet and got half a tablet into the friendlier ones, but most went without. It took a couple of weeks but it finally went away.

I think the ouside cats actually have a better chance to avoid/get over it because they can breathe uncontaminated air. Those of mine that like to be in the cat run got better faster than the ones who stayed inside all day.

With 30 cats, you may have a longer period to live through. Not all cats will get it at the same time; kinda the domino effect.

A drink every night might help! Can't hurt.