Ken,
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.>
John

p.s. my tank is a 45g fw tank with a penguin biowheel 200 filter

Hi jsoong,
You've helped me before with questions and I was hoping you could help once again. Many of my fish have died off in the past couple of months; yesterday a neon, a male guppy over the weekend with dropsy, today i was doing my weekly pwc and found my year old snail's shell empty and it gone. I had 15 neons and i'm down to 4, 11 platys are down to about 5, i lost the momma platy to dropsy a couple of months ago. A female betta was lost to dropsy last month. Could i have a dropsy/bacteria issue? The more aggressive fish seem to be fine; an angel, 2 loaches, a pleco, a catfish, and the frog are seeming to be okay. I use an api test kit (with the test tubes, not the strips) and all the numbers come back perfect- ammonia 0, nitrite 0, nitrate about a 5, ph is at 7.6. Do you gave any idea of what could be causing this happen? Do you know what the average life expectancy is for these kinds of fish? Thank you so much for you help!!!
Ken