For me, it tends to be all or nothing. I quarantine all new arrivals, and generally keep them apart from other species. I bought two hatchet species and a cory this week, and am QTing the hatchets together (as the seller said they were together in his tanks), so that puts 2 20 gallons out of regular use for the 6 week duration.
So far, so good there. But generally, with Characins, I can lose about 10% in the first week, if the entire group doesn't keel over. Most of the time when they all die, that's shipping stress.
It's rare to lose a killie, or a well packed Cichlid.
The Corydoras group are the most delicate. If the shipper single packs, they do great but if they do the old six in a bag, I am not surprised to lose one or two.
When I brought fish back from Gabon, I had 81 fish. A couple of very delicate lampeyes died, and a pair of killies died a few days after arrival. Many of these fish had bounced around in a vehicle for up to 10 days, unfed in equatorial temperatures, in breather bags with daily water changes. They then flew with me (in cargo) from Gabon to Cameroon to Ethiopia, Turkey, Toronto and Montreal before a 10 hour drive to my house. Properly packed fish will survive!
But breather bags cost a dollar each and poly bags cost pennies. Poly is quick, and breathers take attention to detail, and add many times more labour costs. Dealers don't want to single pack, and single packing with poly bags would raise shipping prices through the ceiling.
So my recent buys? Hatchets 20/20 survived. H granti Corys, 6/6, the same for napoensis. With corys like atropersonatus 3/6 and arcuata 5/6 made it. Tetras? Hyphessobrycon rubrostriatus - I lost 2 on 10. I lost one in six N. trifasciatus pencils, with another about 10 days in.
