I'm a fan of Denagard for CRD issues.
It's costly initially (cost about $50 shipped, I could only find it online), but the bottle will last....'forever'. (depends on flock size, but will last my flock of 20 birds a year. Treatment+preventative monthly)
Although it's used off label in the US, in areas it's labeled for chickens, there is no egg withdrawal. I don't toss eggs. That'd be your personal choice, I suppose.
Know that respiratory issues in chickens are generally chronic, carrier diseases. They may 'get better', but be infected and be able to infect healthy birds, pass it in hatching eggs, and can have 'flare ups' again under stress. (molt, cold, heat, etc)
Without specific testing, you won't know exactly what they have, but usually, the outcome/treatment is the same no matter what it is, exactly.
Many people will cull any showing symptoms or cull the entire flock. (the only way to break the cycle and get a 'clean flock' again) But, then care has to be taken to not have the clean flock infected... which means strict biosecurity. Many backyard flocks are CRD positive (some without knowing) and many just treat symptoms and deal with it that way. If your plans for your flock is just eggs and pets, there's no reason to not treat them. If you want to sell chicks, birds, hatching eggs, (or show) etc... you will potentially be selling infected, carrier birds that will infect other flocks. No bird should leave your property alive.
Tylan is your next best bet... but for the money (and results), Denagard tops it by a lot in my experience.
Most other things labeled to treat CRD issues may help *some* with some symptoms, but seem to fail at getting the flock totally symptom free entirely...and staying that way for long.
If you want to treat the flock... I really, really suggest the Denagard, even tho the initial purchase is a bit pricey. Per dose wise, it IS cheaper per bird... and the monthly "preventative" treatment helps keep pretty much all flareups (for us) gone.
