What kind of parasite medicine was it? Was it for external parasites or internal? Did you feed it to the fish or soak some food in the medication and then give it to the fish? Most of the time puffers die from internal parasites which is why you need to feed them something that will kill the parasites. I use Anti-Parasite medicated fish food from Jungle. If the puffs were tank bred then they are normally ok, but if they were wild caught then they normally have internal parasites.
Sounds like some of your problems are from having too many fish in a tank. If its only a 20 gallon tank and you have an angelfish (6+" max) as well as 14 other fish then you are definitely overstocked. Also, puffers are not community fish and should be kept in species only tanks or with compatible fish depending on which type of puffer they were. They definitely should not be kept with angelfish. You can find more info on puffers at
www.thepufferforum.com and
www.pufferlist.com
From your description it sounds like your puffers were green spotted puffs. If so, green spotted puffers require brackish water when young and full marine as adults and they should be kept in species only tanks. Sometimes they can be kept with other fish but they get more aggressive as they get older. They also require about 30 gallons per puffer.
The only puffs that I know of that can sometimes be kept in community tanks are dwarf puffers and even sometimes that doesn't work out.
It would be nice if you could post a list of what all is in your tank so we can help you determine if they are compatible or not.