Featured Fish Dying

I would definitely reduce lighting to about 8 hours for now

 

You could check for an electrical ‘leak’. Broken heaters, pumps etc can put a potential in to the water. Either measure water vs a ground or fit an RCD plug (it will trip with earth leakage fault).
This is a bit left field, but I have seen this before causing strange tank deaths.

 

Have a look at polyclad flatworms, I had one in my tank, came in on the live rock. Took ages to work out why my snails were dying. Then one night spotted it feasting. They can hide in the most minuscule holes in the rocks but when out can completely smother the live stock.

 

What size is the tank?

What fish do you have?

Do you use any sprays in the room where the tank is?

 

Is your tank keeping a constant temperature?. Is it near a window?. Asking as the current heat wave is holding my unheated tropical tank at 78f. If your tank is being hit with direct sunlight, the temperature fluctuations would be greater than some marine fish can handle.