Sorry to hear about the deaths.
the smaller the fish you buy the more chance that they won't survive the move to your new tank. When I gave my breeding pair of angels to my friend I bought 4 small angels and within 4 days they were all dead. I bought 4 more about 3 weeks later and 2 died within 2 days. I still have the other 2 and 2 more as well.
The later ones I brought in got ich within 1 day of purchase. They all looked fine before I bought them and I even had my LFS
keep them for a week before I took them home. IMO
fish have all kinds of parasites on them and only a healthy immune system can truly beat the diseases. If fish get stressed in anyway their immune systems simply don't work to well.
Keep on fishin and I wish you the best.