Yellowing leaves during Veg.

Hi all i know this i a common question but I really need answers to my individual situation and not somebody else's because they may be 2 completely different problems.

So, in a nutshell she is 3 1/2 months old and still in veg and in normal potting mix in 44 gallon drum and with about 25 leaves turning yellow from the outside in.
Yes this includes the older growth but is working its way up to newer and smaller fan leaves, I have tried feeding more often with nutrients high in N and also put up shade cloth to alleviate some of the stress (Its the 5th consecutive day of 42+ degrees Celsius) and was wondering what may be the problem, i am going to purchase a PH test kit in a few days to test that theory because I just water with rainwater from a pelicon (1000L tank) which has started to grow this red algae stuff so this is worrying.

Any help is appreciated

I would say a lil nitro....fish emulsion stinks but it will not burn her up.....use it until u see her get better

It's probably the water, try using tap water that you let sit for a day so the chlorine evaporates.

By all means test your soil, but as growers go, shes getting to old for veg and some strains do shed there fan leaves of depleted food stocks wanting to move to bud, maybe cover her with a dark sack for some 12x12 time or maybe just cloak the lower leaves, to force budding ...its what I hear of aussies do in hot climates to strike bud, consider a foilar feed at days end lower (N) high in (P), (K)