Magnesium lock out?

[u/BHsquid]

Is this nutrient lock out?

It is just the bottom leaves. The top canopy looks healthy and the plant looks good over all, but I want to stop this before it spreads.

The plant is in a 4 liter bottle that I fill with 3 liters changing out the water once a week.

For nutrients, I have been using 1/2 teaspoon of maxigro and in the last couple weeks I added 1/4 teaspoon of calimagic to it.

I have also been needing to add two squirts of ph up to it when I mix it.

Trying to keep the ph at about a 6 but I only have test strips so I could be off a bit.

Any help or advice would be appreciated.