Hi guys! Can you help me about this problem? Is it...

hello, you need to calibrate the ph level to 5.5-6.5, then pour this water to the drain, let it dry if it's earth or just give a fertilizer base and if it's coconut use cal mag✌️

A 7.8 potassio fosforo calcio e ferro fanno fatica ad essere assorbiti....quindi fai un risciacquo con acqua a pH 6.5 e ricomincia a nutrire peace💪🏼

The soil pH is "a bit high" - ??!!?? Yeah, mate - it's way WAY too high and you've locked out the plant's ability to absorb the calcium (and a lot of OTHER nutes). Flush your plant out with 3x the pot volume of pH'd water - so if you're growing in a 3 gallon pot, run 9 gallons of water that's been pH'd to 6.0-6.5 ... and check the pH of the runoff... if the runoff does not get to the same pH as you're running through, keep going until it does. When you're finished flushing, give one more gallon into which you've added 1/2 strength nutes and a dose of calmag since she's going to be very hungry. Then let it dry out completely before watering her again. The damaged leaves are NOT going to repair themselves or look any better than they are right now so don't expect that. What you should expect is that any new growth will not show the same problem. pH EVERYTHING you give - and make sure it's between 6.0 and 6.5 .... Good luck...

That PH is far too high, you've got nutrient lock-out. If you're growing in soil you should be aiming for around PH 6.5 Never, ever give your cannabis plants over PH 7.0!! Tap water is generally PH 7.0-7.2. Adding any nutrients will bring the PH down automatically. Not sure how you got to PH 7.8 but whatever you're doing stop and flush them with PH 6.5.