If you treat with terramycin, do NOT mix with anything but water. Extra minerals (especially calcium) will make any ___mycin or ___cycline medicine null and void. It's really best not to treat with two antibiotics at once,actually. I'd stick with the sulmet.
Incidentally, you're never supposed to start and stop any antibiotic. You do it for the full days recommended or don't do at all. However, the exception is when it might actually harm the bird - as in kill it. Sometimes mixing antibiotics can do that, and the other incident is when they have an enormous reaction to the drug. (rare)
Since you're giving antibiotics, please give PRObiotics daily. Acidophilus from the grover/pharmacy/health food store, Yogurt (only if you're not giving -cyclines -mycins), or a livestock probiotic from the feedstore (see the horse section or cattle section for Probios, Fastrack, and more - must have live bacteria within it - not "byproducts").
Do this every day that you medicate and every other day for two weeks thereafter to help with the diarrhea. It's absolutely necessary to replenish the essential bacteria back into the gut of the bird as that's what digests the stuff the gizard doesn't get, gives them B vitamins and enzymes that digest food (read as thrift and vigor), and keep bad bacteria at bay for the most part.
Runny white is urates and no food and a lot of water. Or drainage.
I'd also use VetRx on their nares (use q-tips and swab their nares and opening in the roof of their beaks - new q-tip for each space) to help get some air into their sinuses and help them breath.
this could possibly also be a virus, in which case the antibiotics won't help but the virus must take its course. That does not however mean that you stop antibiotics. You *must* not stop them at this point.
You can make a glop containing crumbles, the probiotic, some egg yolk to help them heal better. Make it dry or wet as they prefer. Since they're older, you can use a little non-sweetened applesauce to moisten it - the apple pectin is great to help those bacteria you're adding get reestablished.
And yes, it's probably going to run through your flock. Support their immune system as best you can. Honestly, if I were you and had this going on, I'd go out and get some wheat germ oil and use that in their food. Oil based vitamins (A, D, E) are the vitamins that most readily degrade in food because they are oil based.
Putting it in water, etc, doesn't do the job as well as oil based food supplement. A is a super respiratory support vitamin, E is awesome for healing (and neurological disorders), and D doesn't hurt as it helps with calcium absorbtion. I used a cap per four cups of feed, mixed in throughly. I used every other day. Keep food out of the sunlight (as you should anyway).
I'd do that for all of your flock.
If you're feeding a complete feed, that should technically be enough but it's good to have a little extra nutrition during illness. You can't use anything but sulmet in their drinking water while you use it, so the wheat germ oil helps.
