I was told by the LFS owner that Praecox can sometime "shock" easily. I tend to believe him, considering my experiences with this batch and reading the comments of a few other folks.
I acclimated the Praecox over the course of an hour. Yes, there were 10 fish in one big bag, but I got the fish home quickly, opened the bag, and floated it for about 10-15 mintues to acclimate temp. Over the next 30-45 minutes I gradually added small cups of tank water to the bag. Prior to adding any tank water to the bag, I took a ph reading of the LFS water, which was about 6.6-6.7. My tank water is about 6.8, so no big pH leaps really. Once acclimated, I poured the fish into a bucket, netted the fish one by one, and placed them in the tank -- so as not to put any of the LFS water into my tank. I told the salesclerk about my acclimation procedure and he said something else I tend to disagree with -- "you should only take about 15-20 minutes to acclimate the fish, otherwise the collection of fish can raise the pH in the bag". Well, for fish that supposedly "shock", 15-20 minutes seems hasty. And I was slowly adding tank water, so I didn't suffocate the fish, poison them with trapped pH, etc.
I think my acclimation procedure is/was pretty spot on, but I did make some crucial mistakes which, looking back, probably led to this problem and I take the blame for:
1) No, I didn't ask how long the fish had been at the shop. A couple did come from a display tank and I know they've been there at least a month, but the others I can't vouch for. The salesclerk said he took 8 home and they have been fine. Still, a mistake.
2) I had cleaned the tank a little earlier the day I bought them. I broke my own rule: don't buy any fish the same day of a cleaning. Why did I break my rule?
Well, I was due to receive a shipment of plants possibly the very next morning and I knew I wanted to trade in my 4 Angelfish (no longer wanted them) to the LFS before the new plants came, rather than chase the speedy Angels around the newly planted tank. So, I caught the Angels, brought them to the LFS and he offered $8 credit. My mistake was not asking if I could spend the credit at a later date, which I'm sure I could have. Instead, I assumed I needed to spend the credit immediately and so I bought a school of Praecox (a fish I wanted to soon buy anyway).
3) I picked up 3 Rummynose that same day from another LFS to add to my existing school of 4. Those had been at the LFS a while, that I know. But all 3 of those have since died, too.
I'll admit to these mistakes. I won't repeat them and I hope others can learn from them. The ick breakout, which is now visible, is the most likely culprit of the deaths. Plus, maybe the fish were weak. Nevertheless, I thought it odd to blame the 50% water changes, hence this thread.
For now, I'll just let the tank stabilize, medicate the ick, dose a bit of Melafix, and wait a while before purchasing any new fish. I don't think a school of 10 Praecox is unreasonable for a 50g. Hopefully, I can try to build the school again at a later date.
I think I may actually make it one day without a death. We shall see...