Soil?
If you're growing in soil i wouldn't drop the ph down that low. If the water is between 6.5 - 7.5 should be fine.
I'd start with a ph that's closer to neutral first. Then assess how it's progressing after a few days.
Imo be hesitant about adding magnesium at this stage. There might be plenty already there, but if the soil's too acidic it'll make magnesium less available.
Generally a magnesium deficiency will show on the bottom leaves first, then progress upwards. Which it doesn't look that way to me.
Plain water ph6.5 - 7.5, wait a few days. No improvement, try a small amount of Epsom salts as a one off, and see how it responds.
Fix that ph first. If you have a means of testing the ph of the soil itself, i'd see what it reads.
That's what i'd do if it were mine.
Good luck.