Her digestion may be slowing due to whatever is going on inside, the ascites is a symptom of something else going on, a cancer, infection, organ failure etc. It's often very hard to know for sure until they pass and a necropsy is done. Sometimes things will show up on x rays or on ultra sound, but not always. Once a bird develops ascites it's usually a slow deterioration from then on. She may be drinking excessive water in an attempt to help move things along in her digestive tract. It's hard to say. 8 years is a very good age for a hen, some live longer, but many never make it to that age. I'm so sorry, when you love them, you love them, and it's hard to say goodbye. But sometimes that's a kindness. Only you can say when it's time for that.
