I always build up the plants tolerance feeding by feeding and by the week charts also only water when the pots get light but not too much dry back that roots would get damaged or buildup of salts and minerals always water with sufficient runoff seems tho its only at the top of the plant it always starts as if it's an immobile nutrient
You have tip burn and brown leaves that tells me they have been overwatered,the roots have been compromised and the salts are building up because the plant isn't using them. Straight water sparingly and they will like you. Adding something isn't your issue,they just aren't utilizing what is present already. You and others may disagree,just my 2 cents.. Best of luck.