I have no clue what a "normal soil mix for balcony flowers" is but looking at those pics...its either abnormal ph (should be 6.2-6.8 for soil runoff and for the water you use to water with..."around 7".. is a little less specific than ideal) or whatever is in your soil mix is burning your plants. I'm guess ph problem but you'll need something better to measure your ph with to be sure.

PS Not overwatering are you? That will make ph problems (if you have them) worse.
Weedhound Reviewed by Weedhound on . Flowering problems ! ! For the third time i have the same question . Experts help ! Whats wrong with the leafs ? First two pics are from one plant , second two pics from another ! My plants are in flowering for one week . Temperature : around 20-23 C ! Humidity : 45 - 55 % Normal mix soil for balcony flowers . PH : I don't have PH meter so i measure it with litmus paper ! Rating: 5