Didn't the vet indicate how long it would be before the results would be available? Without those results you don't really know what you are dealing with. A usual course of any antibiotic that I have ever given has been from 10-14 days so depending on how long you have been giving them you might not see any results just yet. Also when you put an antibiotic in the water instead of giving a dose directly into the mouth you can't be sure if the bird is getting enough of the med to deal effectively with the bacteria. If the test on the droppings does not reveal anything then I would have gram stain test done on crop and or choana swab of the birds, this should reveal if there is an infection in those areas and if it is bacterial or fungal. If you can find a vet that can do these tests in house without sending them out to a lab your results should be fast, a gram stain test can be done in 30 minutes or less with the proper equipment. How long have they been ill?