Usually you have to isolate that rabbit, assume it has snuffles and contact a vet. Rabbits with colds can be treated, but from my experience, it's very hard and usually culling is a better option. They spread it to the other rabbits, then you can have a barn full of sick rabbits with few treatment options.
For other issues, I have given a rabbits Long Lasting Pen at 0.1ml for every lb. once a day. SQ.
I have not found a good use for tetracycline in water. Works great for chickens...
