Hi again Francine.
Ok I'm going to start off with the most important part of fishkeeping--water quality.
Water test results should always indicate ammonia and nitrite=-0- (no exceptions) and nitrates 20 ppm or lower. The only time ammonia and nitrites should be present is in a new tank set up as in new set up there does not yet exist sufficient good bacteria to consume/breakdown ammonia/nitrites. If you're not already aware of the cycling process, listed below is a link which will explain it.
You mentioned that you have tested the water quite frequently and the results were always favorable. Francine I suspect something went or was wrong with the testing(s). It could have been that the testing eqiupment was faulty or expired and/or the way the test was administered was incorrect. You didn't mention what type of test kit you've been using but the best type to use is the one that uses vials in which a special liquid is added to a vial which contains tank water. Dip stick testing is inferior to the vial method.
What you can do is after testing the water take a water sample to your local pet store and they will usualy test for free. Compare the store's results with your results. If there is a discrepancy then we know either the store's testing is off or your testing is off.
If that's the case then try another pet store for testing and compare all of the results.
Unfavorable water conditions will cause disease and death in fish and no medicine will be able to effect a cure in water that is in bad condition. I cannot stress the importance of water quality enough Francine. If you maintain good water quality you will see not see the amount of chronic cases of disease that you are now witnessing in your tank(s).
Next--quarantining fish. I can see that you really like and enjoy the hobby of fishkeeping and as a result add new fish to the tank(s) on occasion. Whenever adding new fish it's extremely important to quarantine them for at least 2 weeks prior to entry in the main tank. Many fish bought from pet stores are virtually a "Typhoid Mary" who carry a disease(s) just ready to infect a whole tank. By quarantining new fish you will avoid serious consequences. If during the isolation period the new fish shows signs of illness you can either decide to treat it or euthanize it but you'll know to not admit it into the main tank thus avoiding disasterous results.
As for Dropsy, listed below are 3 very good links which will discuss it.
Basically Dropsy is caused by, once again, poor water conditions and can usually be avoided by maintaining good water quality. My preferred treatment plan is to isolate the affected fish, add epsom salt to the water at a ratio of 1 teaspoon per every 5 gallons of water and treat with Maracyn-Two. Even with proper treatment the mortality rate is high once the fish's scales start sticking out like a pinecone. The key to Dropsy is to avoid it.
Acriflavine is also good to use when treating Dropsy but shouldn't be used in conjunction with any other medicine.
You wrote about a possible case of Velvet in the tank accompanied by clamped fins and scratching (aka flashing). The clamped fins and flashing are classic symptoms of a parasitic infection. Try using a medicine called Quick Cure, which is available at Petland Discounts, to treat this but remember my advice about the water quality first.
You mentioned that Moonglow was purchased from a breeder.
Fish purchased from breeders tend to be the purebred/pedigrees of the fish world and as a result they have more outstanding and finer features then their counterparts but as a result of being a purebred they have a tendancy to be more fragile and susceptible to disease as a result of much inbreeding.
Now Francine please do not take this wrong but until you get into the swing of things with bettas and have been able to drastically lower their mortality rate and reduce the problems you have encountered it would be best to deal with regular pet store bettas. Get comfortable with them, learn about their species individual needs and then once you're completely "in the groove" with them should you then move up to breeder bred bettas.
Betta basics are:
1) Constant and stable 80f degree water temperature.
2) Good water quality ( see my earlier comments) which includes frequent partial or whole water changes.
3) Little or no water movement
4) Not overfeeding
5) Varied diet
On a final note. The most fundamental way to maintain good water quality in your main tank(s) is via weekly 25% partial water changes using dechlorinated water that is the same temperature as the water already in the tank and to not overfeed. Also make sure to change the filter media at least every 30 days but don't change the filter media the same day you're changing water.
Hope this information proves helpful and useful.