We almost never find roundworms in the stool. only the eggs. When we see roundworms, it is in the vomit, which is most likely the clear fluid you found. It would be best to have a fecal exam done for all your cats, as this can cause problems for people, but if you are unable to do that, treat all the cats with Pyrantel Pamoate (Strongid) which is very safe and very effective for treating roundworms.