I was checking some email the other night and happened to find this article about a fluid build up in cats called, Ascites. This is a pretty dangerous condition, and a lot of cats do not recover from Ascites. It can be painful too. I'll post a couple of links to the articles I found on the internet. I hope your cat or cats never get this condition. Glad I found these articles.
Understanding Ascites in Cats - Tufts Catnip
Ascites in Cats - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost
Understanding Ascites in Cats - Tufts Catnip
Ascites in Cats - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost