Pond Boss Forums Thread Like Summary - YP Dying - Help Troubleshooting Cause

This year we harvested most all adult fish from our 20 year-old pond, and have embarked on a big rebalancing. The main feature is that we're trying to get YP established, so on May 23rd we stocked 150 6-8" pellet trained YP (plus 50 3-4" RES). See full thread on my pond's revamp if interested in more background info:
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=506290

All perch survived the trip and stocking and we see maybe 50-60 per day during feeding time (hard to really estimate given how quickly they zip around and wander from one spot to the next). The issue is that we still have some perch dying, and have no idea why. I want to figure out why these fish are dying, if it's my fault, and if there's anything I can do about it, because we paid good money for these fish, so it would be nice if a couple actually survived.

We had a steady trickle of YP dying over the first two weeks after stocking. I saw/confirmed 12 dead (out of the 150 stocked). Then, we had no new dead perch seen during week 3. Now tonight, after feeding, we saw 1 perch come up floating, belly-up, and he had the energy to try to swim and flip himself rightside-up, but for some reason he couldn't flip over. I assume he will also be dead shortly, so now we're at 13 "confirmed" dead. This particular perch came up in an area where we had just pellet fed heavily, he had quite good body condition and the blue tint that pellet eaters have, and the school in this particular spot was (abnormally) eating lots of pellets off the top (ours usually only eat sinking pellets). I saw this same belly-up behavior with 1 other perch that died a couple weeks ago.

Questions: What are some potential reasons why these YP died, especially considering the strange behavior of floating belly-up while still apparently being strong enough to swim and try to flip over right-side-up? Could it be that he gorged on pellets and they're just buoyant enough to make his belly/underside be too buoyant? Could it be that he was eating too many off the top and gulped too much air into his stomach? Could I have overfed him, and he died from overeating? Could it be my pellet hydrating technique (hydrating in ziploc bags using well water)? I also squeeze my pellets into globs about the size of a ping pong ball before throwing in, so the pellets from each throw are concentrated and sinking in a tight formation, which means that the school has to be a bit competitive/aggressive, so could this tight formation and "fighting" over pellets stress fish too much to the point of killing one on occasion?

Also, see this thread for water chemistry test results if you think that could be the issue:
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=507377#Post507377

I have seen pellets become moldy when removed from the bag and stored in buckets with tight fitting lids (plastic storage containers). This happens especially in warm, humid weather. This in one of the main reasons to be watchful of older food from production date especially when it is stored in containers with tight fitting lids. Feed in the original bag should have a production date somewhere on the bag/tag. Old food IMO would be more than 10 months old. I have seen feed get moldy when 3 to 6 months old when not stored properly esp in humid conditions.
Having just one or an occasional YP die after 3 weeks is not alarming. A latent death a couple weeks after stocking is not uncommon due to what perch have to go through when net harvested, held, transported, and restocked. The warmer the water for YP the harder it is on their metabolism esp 70F+.