Überdüngung ?

Ok, I'll first measure the pH level. I've just watered with regular, non-fertilized water for now.

Hey!!! Over-fertilization causes burns on the tips of the leaves and they curl downwards. I agree with the other guys, good luck!

Well, you've got a phosphorus and/or calcium deficiency beginning here - and it MIGHT be that you've been overfeeding them since a nitrogen toxicity will lockout those nutrients - but it could also be that your pH is not in a good spot... pH in a soil grow needs to be between 6.0 and 6.5 for the plant to have good access to all nutrients. So since you don't indicate what your pH is but your feeding levels are pretty high (although she doesn't LOOK like she's gotten too much nitrogen), you're going to have to figure out for yourself what's gone wrong here. If it's nitrogen, you should probably flush.... if it's pH, you just need to adjust that level and things should turn around. Don't expect those leaves to improve at all, though, cause they won't... you'll know you've solved the problem if the symptoms don't spread further after a couple of days... Good luck....