Discoloration of one of my planta

Im a new grower ! Ive noticed in the last 3 days one of my white russians has a weird discoloration on its laeaves they seem too look a bit yellow or even darker in some places could someone help me15808839331688376760170559874878.jpg
 
Can u take more pictures, back up a tad
One of them has always looks a bit unhappy which was the darker green plant not even sure if its white russian it came in my seed pot i ordered from dam, sing canna proffesional coco medium, my ph is always between 5.8 and 6.2 maybe its the water im using ? Its straight out the tap and i leave it to sit for 24 hours. Plants are in a 1m x 1m x 2m tent, 15 litre pots with a 600w hps bulb and small carbon filter. I can see to get my temperature any lower than 29.7 degress im starting to feel like its a heat problem. Im using canna A and B nutrients but have only used it once so far. That was 2 days after i noticed the discoloration i thought maybe they needed to start their nutes, also have used rhizo tonic 3 to 4 times with a dilution ratio of 5 ml to 3 litres of water
 

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