I would suspect either Egg Yolk Peritonitis or Salpingitis. Cancers can also cause those symptoms.
I am so sorry to say so, but the conditions that cause ascites are not curable ones. You may be able to prolong their lives some with treatments, but most people will just euthanize since it is painful for the hen when advanced. I usually wait till it is clear they can't do any normal things or enjoy their time with the flock and then cull.
Do you have access to or the funds to go to an avian vet? They could tell you if there are treatment options, but as listed before, they are only bandaid solutions. A regular vet can do the euthanizing part, too, if you struggle with it (like a lot of us do). You can also learn methods on here to do it.