Sorry you lost your pets.
You should have quarantined the newbies. The potential for disease problems occurs with every new fish arrival that may have carried and the older fish got sick and died.
If you have any fish left use a treatment for prevention for bacterial infections. Plenty out there and I'm sure someone here will have one that works well.
Now if you have to buy from a box store please make sure all the fish in the whole place look great. No dead ones anywhere. Then QT them when you get home for a few weeks. I've done it - no hits please

- and qt'd every sucker and made sure they got used to my pond water before they mixed in. they all made it naturally. We won't go into mistakes I made killing some :'( :'( :'(
Good luck.