Having problems with my cuttings/teens

Gardening with cannabis for about two years, still getting the hang of a few things. I have never had a problem cloning plants and raising little ones; I lose the occasional one or two cuts or ween out the weak/sickling. As of lately, I think that my mistakes have been causing the death of my young ones. Im pretty sure this time around it was just early feeding (only raised my tap from 290ppm-400ppm). I lost about half of them, and have to assume that they were just not ready to eat. With that being said I have a couple of questions:

1)How can you tell the proper time of plant production to begin feed? Is there a certain point in foliage production that signifies the plant being ready to eat? I guess I just jumped the gun, the more mature "teens" seem to be doing ok and dealing with the nutrients just nicely.
2)On another note, should you be using ph'd water for fresh cuttings? My tap comes out at 10.0 and I have not been adjusting it as the cuttings have done just fine in the humidity dome with straight un-ph'd water.

3)Another question- when is it appropriate to begin using superthrive? I use it and see a noticable difference in root structure, but dont start throwing it into the mix until I see the first root poke out of the root-riot.

Thats all for now, sorry for the novel; but i may have additional questions as they come to mind lol.

The cutting starts rooting and is ready to transplant. I usually wait 7-10 days or if the plant shows signs of deficiency. This is also when I foliar feed with extremely low dosages. Better to just feel it out. It is ok to use tap h20, I prefer phed because I use a cloning solution. 200-250 ppm is perfect ph 5.5-6.

Not saying this is what it is, but about 3-4 weeks ago my clones stopped rooting as quickly and I noticed what I thought was a pH lockout of some sort. Ended up being thrips