Possible diagnosis (from my limited knowledge):
Swim Bladder Disease
Swim bladder disease is due to a contagious virus, which causes gas filled cysts in the abdomen. As the cysts increase in size, the fish will lose equilibrium, and will show inflamation around the stomach area. Most of the time will be spent at a 45 degree angle with the head down, before the fish loses all control.
The onset of the virus can be slowed with methylene blue or triple sulphur, but will usually prove fatal.
Swim bladder disease can be misdiagnosed if the fish has ingested air, or eaten too much.
Trichodina
Trichodina is generally not visible and it is only once the attack has become overwhelming that symptoms appear. Symptoms include:
Grey film on the skin and gills
Gasping at the surface
Loss of weight and failure to take food
Skin erosion
Rubbing on objects
Trichodinal is often accompanied by white spot and can be treated my the same medications. If appearing as the primary disease, a 10 hour bath in a solution of malachite green at a dosage of 1.5mg to 10L of water is effective.
Tuberculosis (VERY VERY unlikely)
Tuberculosis is a very dangerous, highly contagious disease that can quickly destroy all fish in an aquarium with little advance warning.
Symptoms include:
loss of appetite and severe weight loss
arched backbones
pale colours
tumors and open wounds
inflammed skin
clamped fins
bulging eyes
Teatment is very difficult and often unsuccessful. Once external symptoms start to show, it is usually too late for a cure. All fish should be removed to a bare hospital tanks, with the visibly affected fish placed in a separate bare tank. Both tanks need to be treated with a strong anti-biotic. The original tank needs to be stripped and disinfected. The substrate must be thrown away.
I think you are going to have to stick with until you can get to a good LFS and get some appropriate treatment. Make sure you take a sample of the tank water (and possibly a sample of your tap water) and get a full test done. Also note down all of the symptoms and, if possible, a time line stating what happened when (e.g. x weeks ago did x% water change, yesterday bought two new fish, today fish died, etc.). The more detail the better if the LFS is any good at all. If they are not interested in all of the symptoms and events then it is either they are very good and know what the problem is, or they are poor and will just sell you a general medication - it will be difficult to know which.
Where are you located - there may be members here who can suggest a good shop or two.
Eddie