Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your fellow Nook has heart murmur and fluid accumulating is his abdomen and rear legs.
Is he running a fever?
Was he tested for heartworm?
Did he have radiographs of his heart and lungs?
Was any fluid collected from his abdomen for analysis?
Was any blood work done?
With his symptoms we need to worry about things like right sided heart failure that leads to back pressure and fluid leaking of of the blood vessels in his abdomen and rear legs, liver disease or primary intestine disease which leads to low levels of blood proteins which allows fluid to leak out of his blood vessels, or a mass in his abdomen which changes blood flow returining to his heart and allows fluids to leak out of his blood vessels in his abdomen and in his rear legs.
Likewise we can see fluid accumulate in the rear legs and abdomen secondary to abnormal lymphatic fluid flow.
By testing the characteristics of the fluid accumulating in his abdomen we may be able to narrow down the type of disease process that he is suffering from. Also blood tests allow us to check for heartworm and look for signs of liver and/or primary intestinal disease.
Treatment will be dictated by the disease process he is suffering from. Without further testing I am worried that he cannot improve because we don't know why he is the way he is.
I would highly recommend a complete blood count, biochemistry profile, heartworm test, radiographs of his chest, a urinalysis, and an ultrasound of his abdomen, collecting a sample of abdominal fluid at that time for analysis and culture.
I know that seems like a lot of testing, but he is a sick fellow, and things can be done in a stepwise fashion to look for the most common problems first.
Best of luck with your fellow.