What's in your tank and how many of each? How big is the tank? What are ammonia, nitrIte and nitrAte levels? What sex are the gouramies and which species? How long have you had them? How did you seperate them? What signs of disease does the ill fish have other than lying on the bottom? - does it go up to breathe regularly? How's it's appetite? Does it look bloated? What color is it's poo? Any fungus-like growths, ulcers, red streaks, inflamation or spots? How's it's breathing - are the gills moving abnormaly rapidly? What do you feed them and how often? What temperature is the water at?
Try feeding a de-shelled crushed green pea. If the problem is bloating/constipation, this might help. It can also do something for swim bladder problems (though this is unlikely to be the issue - gouramies rarely get swim bladder problems). Feed only green crushed peas or frozen daphnia for a few days and see if that helps; meanwhile answer all my above questions.
