Leaf Septoria?

Put asap CalMag,wait for 3/5 days and see how your plant is doing,if it doesn't get better maybe your water ph is not correct and maybe your ph the soil too,or you have nutrient lockout.You thought about it? Your leafs are dark green wich you are using N little bit more if you have nutrient lockout just take a lot of ph water and flush your plant,wait to dry and then add little nutrients and gradually go up(YOU CAN DO the same with less water,just enough to you wash off the SOIL if your PH in the soil isnt correct. Hope i help!

Imbalance in ph during some watering, small aftermath, calcium issue, normally not a biggie, mark of ex mistake, unless it starts to progress ... Check ph meters.

Yep lacking calcium.

no, lo que dice @m0use es correcto. ve por ese camino y asegúrate de no estar sobre alimentando con nitrógeno, de ser asi, es la causa de no poder absorber el calmag que necesita. Si estas en flora y estas usando algun alimento con nitrógeno, yo lo dejaría de usar y agregaría calmag como dice m0use. Suerte

looks to be the beginning of a calcium deficiency. I'd add in some CaMg asap. Or fix the ph is calcium is in the medium but can be absorbed. Good Luck!