Some people use Baytril prescribed by a vet, and it is pretty powerful compared to feed store antibiotics, although it is not legal to use in chickens for eggs or meat. Enrofloxacin, available online, is very similar, and if you do a search at the top of this page, you should find some sources...
Welcome to BYC. Casportpony and some others have a few threads about ascites if you search ascites at the top of this page. I haven't treated it, but I have read that it can be from either heart failure or from liver failure secondary to egg yolk peritonitis and internal laying. Draining at...