Fish Lifespan question

[KenW]

I got about 15 neons about 8 months ago, and I'm down to 7. I found 2 dead, or what was left of them before the other fish and the snails got to them, but no signs of the others over the last few months. I had about 9 platys, am down to 4, but never found any of them. Momma platy got dropsy and died in December, and this week we moved out a female betta that was in this same tank & quarantined her to a fishbowl, also with dropsy. The rest of the fish are new or going on a year old.
My tank is a 45g fw tank and the water is well oxygenated with a 12" bubble wand. All the chemical numbers are great, currently a little high in nitrate at around 20, but the rest come out great; ammonia 0, nitrate 0, pH is 7.5. I change the water weekly, by about 25 - 33% and vacuum the substrate as well. I feed them sparingly, enough for them to consume before much of it hits the bottom. But they get fed almost daily. I am wondering if the frozen red worms i give them as treats about 2 or 3x a week could harbor some kind of parasite, even though it says its been sterilized. ????
So why can't I get some of the fish to live more than 6 months? the frog, pleco, catfish, ramshorn snail, & male platy all survived an ammonia bloom(?) last year and like the rest of the fish, all seem to be fine, but these other random fish just up and die for no apparent reason.
What is a fish's lifecycle?
Any suggestions?

[SWerner13]

I don't know the average life span of those fish, and I'm sure each is different but there are number of things to keep in mind. From my experience working at a petstore and owning them myself, neon's are very "finicky" fish. They seem to drop off with no rhyme or reason sometimes. As for the platy's you gotta keep in mind what these fish go through to get to a fish store. They are grown in a farm in a tank that is over stocked. They are then shipped to the store and tossed around, thrown in the tank without being acclimated very well, and then still over stocked. They just aren't as likely to be hardy. I think your frozen food should be fine. These are just a few comments.

[ellisb1]

Did your neons show any signs of distress? Discoloration, color fading, restlessness, strange swimming habits, fin rot or spinal problems? I wonder because neons can die with little warning because of neon tetra disease. Other fish can catch it also. I am not sure, but I figure that the average lifespan of a neon would be about 2 years and about the same for platys.

[Zagz]

Neons can be very weak from some lfs's. Have you purchased them all from the same store? If so try another store.