From looking at the plant, it’s definitely a mobile deficiency working it’s way up the plants fan leaves. If the water is new, and that’s the only new variable, it’s definitely off-set something.
I were you in this situation, I would Ph test the soil itself to see the true level (might be a little high), and play around with the Ph during watering. For ‘P’ intake in medium, it’s best around 6.5-6.7 from what I believe, so slightly raising Ph may help give it that little extra bit it needs. Every time I water I change the Ph level slightly within the right ranges, as different nutrients are absorbed better at different Ph levels.
Just my 2 cents towards helping it back to a thriving green state, I left a chart I use to help me remember Ph levels for different nutrients if you’re interested, hope this helps some.
cheers bud!