I am not a tropical expert by any means ... I am more a goldfish person ... so take this with some grains of salt!
Snails (apple?) have a lifespan of 1-2 years depending on water temp. Neons can live 5+ years, but they are fairly finicky (a/c my sister inlaw) and don't live nearly that long. <Some sites says average lifespan is 1.5 years.>
Your basic water parameters appears fine. If you are losing a fish every few weeks, it is likely to be from natural causes. Dropsy is just a non-specific sign of end-stage organ failure, as likely from old age as from an infection.
I don't think your losses are bacterial unless they are in clusters. One exception would be fish tb or internal parasites. <At least among goldies. Serious goldies keepers will do autopsies on their fish to look for this to decide on how to treat the rest of the fish.> If the rest of your fish are happy & appears healthy & growing, my tendency would be to just watch. <I hate to treat blindly with meds.>
I need help with figuring out why all my fish are dying. Three days ago, I did a 50% pwc to lower the ammonia levels. They were at 2.0, and at the same time I added more substrate because I was told by my LFS I didn't have enough in there. Yesterday I had 2 fish die, this morning there were 3 dead fish, so I did more tests and still the ammonia is 2.0 or so. Ammolock is having no affect. I added stress coat and Prime when I did the pwc. I have a API test kit and my ph is very low at 6.0(how can i get this number up?), kh is 80, gh is 60, nitrate is .1 and nitrite are .5 still after 50% pwc. So, with more fish dead this morning I did a 25% pwc, and now my 5 neons are floating around upside down and sideways and will be dead before too long I am sure. Plus - its reeking of sulphur that I thought was from the Prime, but its never stunk like this before. The 2 platy's seem okay, the beta is okay, and the pleco as well, the 5 zebras seem okay. I have had the tank for 3 months, and posted previously about cloudiness and was told I was still cycling. I unfortunately started it by cycling with fish in July and about 3 weeks ago it completely cleared up and has been so beautiful, then i did the pwc and added the substrate and now everything is dying. the tank is 40 gallon freshwater. I had hornwort plants in there but took them out yesterday becuase they were dying as well. I guess I should prepare myself for everything to die off, and let it sit for a few weeks before adding any more fish. But my main dilemma is the ammonia. I cannot get the number down!! I added the plants to help initially a couple of weeks ago, hoping it would help with the ammonia. I have been doing a 50% pwc on sundays, and 20% pwc on wednesdays to help the situation for the past 4 weeks. I feed them every other day and only as much as they can eat in 2 to 3 minutes. I just took 2 dead neons out.
I am going to move the beta to a fish bowl...
set up a 40g freshwater tank. Have had many tanks in the past, but am feeling very amateur!