I would try and up those lights off temps up a bit mate, around 20-22c would be ideal, and would shorten the on/off temps swing. Looking at the plant i do not see the usual signs off heat stress ie sidewards rolling of the leaves to retain moisture, nor do i see any upper bleaching, burning, or discoloration other than the obvious ones. From memory coldish temps can play havoc with the uptake of P,K add is a slightly too close to lights on top and this could be causing the demand for more p,k...
I dont think nitrogen is a problem here, more p.k The leaves have a nice green look to them from here and considering these leaves have turned yellow upper to middle of the plant i call for P,K i can also see a fair few pinky, purple hues to a few leaves.
Just increase your bio bizz feed? is it bio bizz? If so these bottles are meant to be used together, ie 1.5ml grow, 3.5ml bloom. Hell a nice splash of black strap mollases wouldn't go amiss. You can find the right stuff from a company called meridian they have a wicked jar for around 3 quid and will last you several grows. This mollases stuff is really kinda like a organic boost and also gives your microherd a lovely meal that will have them going nuts.
Best of luck mate
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