Black colouration within Phal leaves. Virus? Magnesium deficiency?

[Clay_pot]

This discoloration appeared on one of my NoID Phalaenopsis at the beginning of July, it is only on the first new leaf of grew this year. It then appeared on another NoID phal about a month later. I purchased them both at the same time in January, so they are from the same shipment. It hasn't spread to older leaves, or my two minis, and has only appears on the first new leaf from this year, on both plants, the newer leaves dont have it yet.

I've looked through a few resources and think it js either a virus or a magnesium deficiency but I have next to no experience with orchids. Anyone have any thoughts?

They have an LED light 10 hours a day, water with city water that sits out for whole night. Soaked usually for 15 minutes at most an hour if I forget them. Fertilized with 15-5-5, biweekly.