I am pretty convinced the discus died from low O2 levels. These fish were very healthy and I had them for over a year doing great. They don't all just suddenly drop dead the day after something new was introduced to the tank.
The few fish that did survive were gasping at the surface when I came home. Keep in mind one discus did survive. I lost 3 adults and 1 juvie discus, a few shrimp and a german blue ram. My neons and the discus cull made it through.
If it was chemicals that leached into the water I am pretty sure all would have died. So I am ruling that out. Also since so many have had success with this company and since it's just sand blasted wood I doubt it was chemical.
If I were to try this again I'd soak it, boil it if possible. Do large water changes or keep the fish in a quarantine or run air stone . Monitor PH for sudden drops, test O2 levels and most importantly do it on a weekend when you have time to monitor your tank for a few days.