At first I thought it was light burn and moved lights higher up (2×SP250's), now at 1m (3ft).
Soil mix.
Ph water to 6.2 to 6.5.
Is it mag deficiency?
Why so early on?
About all you can do is ride it out. I wouldn't re feed for a while. You can re pot it in to a bigger pot and water as normal. It may help to dilute what you have. But unfortunately the damage is done.