Leaves rapidly turning yellow .Please give me some advice.

[camille1585]

First of all, welcome to OB zeljka!
It's hard to tell from the pictures, but how many different phals are doing this?
In the fifth photo, if the phal only has that one leaf that's yellow, I don't think it is a problem as it appears to be the olderst leaf. Phals regularily shed their olderst leaves.

Usually, the first thing to do when you notice problems is to check out the roots, as root rot can lead to leaf yellowing, among other things. But if all of them are turning yellow at once it could be other things. Did they get overly cold one day/night?