What could cause the appearance like this?At first...

5 is too low of pH. it'll definitely cause soem issues if swinging down that low. is the water touching any solid substrate at top. if it's soaking up water in the basket vs a gap between basket and water, put a gap in there... if constantly wet, someties see leaves like that. as far as dosage.. this is calculated from guaranteed analysis labels and does not include your what your tap water contains: 120-130 n 50+ P 190ish K 100+ ca 50+ Mg (i'm closer to 75, but from what i read most plants only need 50ish. this could be why several months into grow occasionaly some plants get a bit too dark - find out over time one way or another.) less than 110 S (this one i have a hard time reducing while keeping other stuff in the range i want, due to the products i use, so i find it best to describe as less than 110) 1.3-1.5 EC is a good place to start... these ratios ramped up or down in this range will result in supremely healthy plants all the way until harvest. you'll always have to do some adjustment compared to others. Environmental factors vary, so effective dose will vary.

Ph is not stable if it is 5 to 6.5 yeah big issue there. .1 in ph is a huge difference for the plant. I usually run a 5.8 so I got room to drift a little, and not hurt the plant. Wrong nutrition amounts will cause big ph swings, and or it needs changed.

If your writings are correct an EC of 3 IS way to much. Go easier.