Milky looking Blue Pearls

My shrimps seems to turn milky white in the middle and with time they turn more milky and dies off after a week or so and I've been having at least 1 death a week.

Does anyone know if leaving the lights on your shrimp tank for too long harming them or stressing them out as i've read it somewhere online and how long should my lights be on daily?
I've got 4 shrimp tanks and 3 of them are fine but my Blue Pearl tank. The water parameters are fine.
Here are my water parameters.
pH 7.4
gH 5
kH 4
TDS 247
Copper 0
Nitrine 0
Nitrate 0
Ammonia 0
Temperature 72-75

I use to leave my lights on from 8 am till 11 at night and am wondering if that caused it.
I was also told the thinkness of my gravel might have something to do with it and now I have half an inch of black sand covered with 2 inch of black gravel.
Someone told me that it might just simply be stress and I cannot find out what is stressing them.
I've also started adding water slower in the tank so it doesn't give them a shock from the temperature change.
I've also been told to add trumpet Snails in the tank as it might be a bacterial infection and that the snails will clean the gravel to keep the tank cleaner.
I do at least a 30% water change every week using syphon filter and I do try not to disturb the gravel too much.
I've also added some Catappa leaves in there.
Am not sure if this will help you guys understand whats going on in my tank but I've also got christmas moss,java moss and Anubias and also another plant which i do not know the name of.
Theres a driftwood with some clay shrimp caves and also a double sponge filter with an aquaclear HOB filter.
For water change i do around 75% tap water + 25% Ro water and so top off I use 100% Ro water.
And I do have my 50 Gallon HOB on the lowest flow for the water flow.
Am still new to the hobby and I cannot figure out whats wrong with the tank and its starting to get stressful as i've already tried everything online.:icon_sad:
Thanks all in advance.