Remember, when treating via the drinking water that the dose should be adjusted depending on the volume of water consumed. The recommended doses are for mild temperatures. Use a little less in hot conditions, and up to twice the dosage in very cold weather when the birds drink much less. Avoid using drugs at lower than recommended doses and for less than the recommended time. This practice allows the organisms to become tolerant or resistant to the medications by being exposed to them for insufficient time or at levels too low to elimimate them.
Antibiotics prevent nothing.
They are used to treat certain infections but, by no means, prevent anything. Avoid mixing medications unless the mixutres are shown to be safe. Certain combinations are known to be safe and generally used. Don't get carried away and assume that all medications can be mixed and dosed simultaneously. Proceed cautiously and try to stick to known safe combinations.