Well the sand is from a lake so I doubt it’s inert. Lot going on here and a lot of new stuff in the tank. Not a good idea to chase the ph. Better to find out why it’s rising? This being a new tank it may still be cycling. Dead material inthe sand may have caused some sort of bubble of bad air coming out of the sand and killing the fish. Next in all pics your discus are showing their stress bars. They don’t look happy. If it was just figuring out who’s in charge some wouldn’t have had lines. Tank looks great but you’ve got a lot of sand and dead spaces. Not good in discus tanks.
As to your plants. They were grown emersed. Roots in water leaves above. This is what seller’s do. They look nicer this way. The give away is the round leaves. Once submerged these leaves will die off for long straight leaves. This will take time. Swords are huge root eaters. So root tabs do help. But again now that’s something else added to that tank. Best to grow these out in another tank or look to buy submerged ones already.
I wouldn’t start adding those leaves till you get the tank stable. You’re heading to a dark water tank. People usually have this type of tank stable before adding discus. Discus are known to need clean water not fluctuating water. May be best now to pull your discus into your qt tank and get this main tank stabilized.