Some basic information for you - first it is important to differentiate betweeen pleural effusion and pulmonary edema. It is generally pretty easy to do this with x-ray so you may wnat to clarify this point with your vet. With pleural effusion, the fluid accumulates between the chest wall and the lungs and the treatment is to do a chest tap (put a needle through the chest wall and draw out the fluid) which usually gives immediate relief. The fluid is then analyzed to try to figure out what was causing the collection of fluid. Sometimes this analysis doesn't give an exact diagnosis but just narrows down the list of possibiilties and additional tests need to be done.
With pulmonary edema, the fluid collects within the lung tissue itself. From there, sometimes the pattern of the fluid collection (whether it is in only one lung lobe, if it is more around the base of the heart, etc.) can give you a clue as to what could be the underlying cause. From there, how this is treated will be determined by the cause of the fluid accumulation. Diuretics are the main treatment if the fluid accumulation is related to the heart (often in combination with drugs that cause the blood vessels to dilate and sometimes with drugs that increase the strength of the contractions of the heart).
Another important thing to know is that even if the heart appears to be normal on an x-ray, that does not mean that it is functioning normally. An x-ray only lets you see the outline of the heart - In order to evaluate things such as whether a certain chamber of the heart is enlarged and how well the heart is contracting an echocardiogram (ultrasound of the heart) needs to be done.
Because your dog has not responded to the diuretic or the antibiotics, I am concerned that there is something more serious going on. Things such as atalectasis (collapsing of part of a lung lobe) or even a mass within the lung can look very much like fluid within the lung and these have different treatments than either pulmonary edema or pleural effusion
At this point, I would recommend that your vet send in the x-rays to a veterinary radiologist so that he can have a specialist look at the films. They will often be able to pick up some very subtle changes on the films that general practitioners will miss and be able to narrow down the list of things that could be causing this coughing and fluid accumulation. They will usually make some recommendations about whether additional tests should be considered in order to get to a more specific diagnosis.
So, to specifically answer your questions, the most common treatment for fluid in the lungs is diuretics (sometimes combined with other drugs) but there may be other treatment options depending on what the underlying cause. It is important to try to find out what that cause is in order to provide the best treatment.
I hope this information is helpful. If so, please click Accept. If you have additional questions, please let me know.