Please Help with my snails and corals

[kenman999]

Recently I have added a torch coral, xenia and 2 giant turbo snails to my 60 gallon tank. Everything was dripped for 3 hours before I place them into my tank. Everything was open and alive within the 3 hours. However as soon as I place my corals and snails into the tank, the torch and xenia changed to a dull blue colour and the torch started to spit out some white mucus. The snail went inside its shell not moving at all. After 24 hours the snail was still in the same spot. I picked it up and it smells fine. The torch was still shrunk and dull blue. After 2 days the snail died and my xenia died too.

My tank is 4 months old with 50lb of liverock with livesand

water parameter(I use RO water)
ph-8.4
nitrite-0
nitrate-5
ammonia-0
copper-0
calcium-420
phosphate-0
salinity-1.024( with instant ocean salt, tested by a refractometer)
kH-9
temp-78

I use APi test kit and I have brought my water to two LFS and both results are the same.

I have 6 t5 blub, 4 day light and 2 aqua blue+ turned on 8 hours a day.

Torch on my sand bed with moderate water flow.

Fishes- 2 clownfish, 1 firefish, 1 yellow tang, 1 scooter blenny, 2 cleaner shrimps. All doing perfectly fine for 3 months.

Why are all the coral and snails dying? Please HELP Thank You.