Well, there certainly is something going through your fish, moving fast enough to infect and kill them this fast suggests bacteria rather than parasites. (Though both may be present)
I see a couple of options:
1) Try treating with whichever antibiotics you can, and if this does not take care of it sterilize the tank, and get a different antibiotic for the new fish. If this antibiotic does cure them then EVERY TIME you get new fish quarantine and treat. NEVER add new fish directly to the main tank. Get a UV sterilizer fro the main tank.
2) Let it run its course.
You may lose all the fish. When you want to restock have antibiotics on hand, and treat all the fish as you get them, in a quarantine tank. You might decide to treat with both antibiotics and anti-parasite medicines while the fish are in the quarantine tank. (really good idea)
Get a UV sterilizer for the main tank.
There may be more than one thing going on here. The fish may be stressed from several things, allowing one disease to finally take over and kill them. Here are a few possibilities:
Mycobacteriosis: A disease similar to tuberculosis, and sometimes called "Fish TB". It infects the fish, but does not usually kill them fast. They even grow, and seem OK for a while. However the infection weakens them and they are more susceptible to any other disease or parasite that may also infect them, and they are more sensitive to changes and stresses such as the chase/capture/bag and new water conditions that happen when they move from store to home. They went through similar stress in moving from the hatchery to the wholesaler, then from the wholesaler to the store. The accumulating stress and low level infection leaves them very vulnerable to infection by other bacteria, parasites or anything else.
Stress from acclimation. When the water chemistry is different through all those moves the fish need time to adapt. If the changes are too extreme they cannot adapt, and they die. Make sure the fish have been at the store for at least a week, longer is better, and are active and eating. Test the TDS, GH and KH in the water from the store (in the bag when you buy fish) and make your quarantine tank match the store water as closely as you can. If you cannot match it, then slightly harder water, slightly higher TDS, GH and KH will be better than softer water. Fish can acclimate to harder water easier than softer water. Then drip-acclimate the fish. While they are in quarantine, being treated for whatever diseases and parasites they are bringing with them slowly change the water chemistry to match your main tank. This may take a month or more.
Intestinal parasites. Many fish (especially bottom feeders) bring a light load of parasites with them. These do not stress the fish as long as conditions are pretty good. When other stressful things happen, though, the fish immunity has a harder time fighting off the parasites (and all the other diseases) so the parasites grow faster, can reproduce, and the fish may pick up a larger load of parasites, or more than one species (especially if they are kept with a central filtration system for even a short while). While the fish are in quarantine, treat for internal parasites.
Other generalities:
Do not buy inbred or line bred varieties of fish. The specialty colors of Dwarf Gouramis are an example. To fix the genetics of that particular color the fish are too closely related, and the gene pool is so small that any weakness at all is also concentrated, so the fish are not as strong as their wild ancestors. "Balloon" species are similarly inbred, and also have distorted internal organs, and often misshapen spines. Painted, dyed or tattooed fish are subjected to toxins and stress that can kill them, too.
Look at all the tanks in a store. Ask about the filtration. If the fish you are interested in are on a common filter with some diseased fish do not buy them. If you are willing to quarantine and prepared to treat them, and the fish you like look fine, it is a risk you are walking into with your eyes open: You know the fish have been exposed to whatever the other fish have.
If you can find a local breeder of the fish you like you will likely find healthier fish, and may already be acclimated to your water. Often aquarium clubs are a good way to meet local breeders.
38 tanks, 2 ponds over 4000 liters of water to keep clean and fresh.
Happy fish keeping!
