I recognize this is an old post but I saw a much newer post referring back to this one. I'm new to owning chickens (less than 1 week in) but am very familar with Ascites in humans. My mother recently died of Ovarian Cancer. Ascites is unfortunately common in cancer patients. While chickens are not humans, perhaps there are some similarities or info that can be gleaned from what they do for humans.
First, the ascites hurts -- the more there is, the worse the pain. Essentially the fluid buildup pushes against all the internal organs and stretches the skin -- it is extremely painful (for humans) and drainage is critically important. I'm not sure how or if chickens can communicate if they are in pain but humans can and its painful. Removing the fluid is essential ... drainage is one method and another is via diuretics -- however, diuretics put strain on the heart and it can easily result in dehydration (because thats essentially what it does). Folks on this thread have talked about drainage by needle -- I highly recommend this. I can tell you from watching this first hand experience, the pain of a needle inserted into a human's abdomen was WELCOMED relief for my Mom because it immediately removes the pain.
For cancer patients, one of the main causes of the ascites is a
drop in Albumin -- this is one of the main proteins in your body. As your albumin drops, your cells are unable to keep fluids in the proper space so it begins "leaking" (i realize this sounds really weird but this is actually how the doctors described it to us). This is also called "third spacing" in medical terms -- your body begins swelling and fluid collects in cavities (your abdomen being one of the key spots where fluid can collect). Note -- a drop in albumin is one of the key measures hospitals and doctors can use to predict whether you will live -- its that important. Its a measure of your overall health in a sense -- or ill-health said better. It can be improved by better nutrition BUT when you have cancer, your body's ability to remain healthy and to make use of nutrition is severely compromised.
For humans, there is an albumin shot -- its expensive and its only temporary. Organically,
non-denatured whey protein (Whey Cool is a good brand, organic, and purchasable on
Amazon) has been illustrated - via scientific studies - to help increase albumin BUT i'm not sure chickens can be given this (although I can't imagine why not). Additionally, this helps with nutrition BUT if the underlying cause (e.g. cancer or perhaps as some have noted liver or heart failure) is addressed, its unlikely to do anything but help.
For whats its worth -- and please understand I know nothing about chickens -- if there is a chance that the chicken is feeling anything like a human does, I would do anything you could to remove the fluid as the pain is very bad.