def mg deficiency...
i'd fix that before assuming anything else... proper amount of mg will restore a lot of lushness on its own.
i don't get why potassium is being mentioned. you'd see stuff forming along leaf tips before anythign else with that, which is not occurring. So, if oyu see it progress where the serration on teh leaves are all burning at the tip or necrotic etc.. then you can consider low potassium as the culprit.
i see start of interveinal with some spots.. that's mg. the thing about Mg is that when you see the symptoms, the deficiency actually started 30+ days ago. The time frame is sometimes usfeul to consider....
low p would come with symptoms too that would stand out, which i don't see here... i'm not sure what the stimulus is for believing it's low if there are no visible signs to go off of... i'd love to learn what the reasoning is despite no visible symptoms... must be intuition?