I really think that it's a problem with the shipping of that concrete species and not a thing with my sand. So you may be into something.Hi, I've just skimmed the thread but I see they were sent overnight and then they had a long acclimation. I don't have time at the minute to go into detail but if the others think I'm right then perhaps one would clarify, but this might be the problem. Overnight shipping and then a lengthy acclimation with the bag open to the air might be the issue. The fish pollute the water but it isn't a problem until you open the bag.
The last shipment was three days ago. Two have died, another two looks sick, and the other four are more or less ok: not super active, but searching for food in the sand as a group, as cories do. I don't think any problem with the sand would have killed those two so quickly.
They must be really sick when I introduce them into the tank, or some chemical of the tank is affecting them that doesn't affect the other fish.
I'll see how the ones that are ok fare, if I observe that they are getting sicker, I'll scoop them out and put them in a tank without sand.