Help needed please

Hello everyone iam new to the forum and have not long started my 1st grow and iam having a few problems i hope one of you nice people on here could help me with. I choose pineapple express auto and theyre 25 days old now under a 20/4 cycle.They were all fine up until 1week ago and they started to go yellow on the leaves and some of the leaves on one plant in particular looked to be dying, and has stopped growing. The other 3 seem to be ok but i have noticed some yellowing on the leaves today and iam getting concerned! The ph level is 6.9 water and 7in the soil. i think this could be the problem but i dont really know how to rectify it so any help you could give me would be very much appreciated!!!! I have put a few pics up to let you see.

kinda looks like nute burn. whats your ppm, brand of nutes, an soil. what your gonna have to do is flush your plants with 2x the amount of water as your pot so 1 gallon pot run 2 gallons of water through, let dry then pick up where you left of. have a growtastic day

Id do what facepalm said

thx guys. I'm using BioBizz Grow+Bloom AllMix Soil.I think my water ph level was off, i was using colour bars to gauge it. Got a ph meter and ill flushwith exactly 6.5 water and go from there. Sound good to you guys?

Also when i go back on BioBizz i will adjust my water ph after i add the nuts. Does everyone do that? If so how? Never heard it after reading the forum quite a bit. Goes without saying i guess?

sounds good rob, phing is important. most nutes have buffers that keepit around 6.0s some work better than others. 6.5 is good for soil. always adjust after you add nutes.

Before you go flushing all ya nutes down the sink...... Just use plain old water for the next 2-3 feeds. 9 times out of 10 that will cure mild nute burn unless you have been hammering the nutes for a long time and not taking notice of what your plant is saying to you.

Thx guys. My problems were showing up even before i started the nuts(yellowing off leaves then the edges started to curl up, go orange then die) so I'm pretty sure the problem was my ph levels. I Flushed 2 days ago and soil is pretty wet still so when that drys out I'll add water +nuts again. 3 of my 4 plants are still looking healthy and growing but 1 hasn't grown for about a week, hopefully he'll spring back into life any time now. Ill upload some pics shortly gotta go to work now. Thx again for the reply's.