Black tips on leafs

So my plants were super healthy. Then all the sudden every leaf now has black tips on them and they feel dry and crumbly. I dont use nutrients much other than just watering them. (Keepin my first grow simple) my wife recently put some of those white fertilizers in the pots when she did the garden.
Sometimes i use a organic kelp fertilizer. Only twice now. Anyone know why this is happening or what causes it? I picked out all the fertilizers she put on the soil. Not sure if that was the reason but it happened almost immediatley.
I water them once the soil is completley dry too.

Thanks everyone!

 

Nope its warm. 25°C.
Also here are some pics.

 

Thanks for the tips. I hope its just nute burn. Def not over watering.