BUDGIE L0V3R

Hi, I have an aviary with about 20 budgies and they have caught some kind of disease, I am not sure what it is. They have diarrhea and end up dying in around 1-2 days of being sick. The aviary has a grass floor, i gave them some vetafarm cocci vet and that cured them after several had died. I thought i had gotten rid of the disease because they all came better but now they are looking sick and have diarrhea again, I have separated the few from the rest of the colony and put them in a nice warm room. What could it be and whats the best way to stop it? Thanks all, please answer quickly