Today I cheked the NO3 and PO4 levels and it was a bit out of normal .
NO3 : 1-2 mg/L
PO4 : 4-5 mg/L
I think I have to change my dosing routine. Those curling leaves probably are related with high PO4 levels.
If anybody can advice something useful please let me know, I'm very curious what could be the reason of my problem.