Need help, nutrient, light, or ph problem. PICS attached

I live in a hot (scorching tropical climate) 100 degrees inside without ac - and currently have some plants in soil in veg under a 175 watt hps indoors.had it closer but now keeping it at around 14 inches...scared im givin my babies excessive light... Im doing it in the bathroom cuz its the only room that has air conditioning...keeping the room around 73-75 with a fan blowing the cool air onto the lamp and plants....i have one of the walls of the box open all the time see pics....box is 2x2feet....the only thing is my plants look healthy but some of the leaves keep turning yellow and dying....the ph of the water is around 7.5 and it is a little hard because it comes from a well...i tested the ppm one day and got 211 the other day 204. i originally planned on doing hydro so i had some ph up and down so thsee past few days i have lowered the ph of the water to around 6....should i be doing this? also i have one of those air conditioners that has one part inside and the compressor outside...i keep the room closed almost always 18 on 6 off light schedule...could it be that these plant are missing air exchange? would the ac help air exchange? help im pretty new at this...also i put 2 of the plants outside to flower and they do not seem to have the same symptoms...the main reason i want to veg is to keep some mothers and clones...i have a 400 watt hps for flowering too but might just flower outside because there is about 12 hours of light here year round....ive got lemon skunk, crazy miss hyde, some sativa strains, etc...bix mix but mostly sativa indica hybrids.