Can anyone please tel me what age/size the plant needs to be to start bein fertilised

im usin canna coco a and b mix and i fed it her on like day 4 is that wayyyyy too soon? ive been flushing ever since but my water is ph 7 (ive just todAy bought some ph down to make it 5.8 tho)! i only put a small amount of fert in th watering can and by week 2 ive lost 3 leaves and another is turning yellow round th edges, these damaged leaves are from th bottom first set and th next set up the rest of th leaves look a really healthy colour but are abit twisted what am i doin wrong?

is it ph thats fucking th leaves or nutes?

any advice wud be greatly appreciated