The information I found said to use 1/8 teaspoon powdered Tetracycline per cup of drinking water. There are 16 cups in a gallon. So, I think you should use two teaspoons Tetracycline in a gallon of water. However, be advised that Tetracycline is one of the most commonly used antibiotics, so many strains of bacteria have become resistant to it. Therefore, it is often better to use other antibiotics, such as Tylan 50 or Oxytetracycline, as they will tend to be more effective in treating diseases. But I suppose it won't hurt to try Tetracycline first.
It is unlikely that they will get egg bound while they are sick, as egg production, like you said, is slowing down. A decrease in egg production is common when chickens are stressed out by disease, heat, other birds, and other external/internal factors. Just keep a close watch for any egg bound symptoms (penguin-like appearance, lethargy, straining, etc.), and your birds should be fine.
