Your plants are way to far away from the 400 watter.
You need to let tap water sit and breathe,lid off,for 48 hours imo.
Any good one part nute will bring down the ph levels to the correct levels and if you need to increase it and cant afford a bottle of ph acid use lemon juice by adding a few drips per litre until you get the correct reading.
You will need to buy a ph test kit.
I would say your main problem is that your mh ballast is way to far away.
I have my 400 8" above my plants after they are 2 weeks old starting of at about 24" when they are newly popped seedlings and lowering by an inch or two a day.
Best of luck