Sick DWC seedlings PLEASE HELP!!!!

So my seedlings are about 2-3 weeks old and everything was going good until I bought a ppm meter. When I did I found that my tap water PPM was 800 @.5

Even tho my plants were doing great in the 800 PPM tap water I thought that 800 was way to high and so I bought some RO water and changed the rez. I added some floranova grow till the water was 370 ppm. Then the next day the tips of the leaves started going brown and drying out and breaking off and the plants as a whole are starting to turn light green/yellow. Since then I thought it was either nute burn or mag/cal def So i changed the rez again and added floranova and mag/cal+ till the PPM was 315. The problem is still getting worse.

There are 6 plants all different strains, 3 in one rez and 3 in the other. Weird thing is that 2 of the plants are completely healthy and in opposite rezs.

Even though the plants don't look to healthy up top the roots are stil growing about 1/2 an inch everyday.

Anyone please help me!!!! I don't want to lose these plants.

Also PH is 6, water temp is 70, air temp is 73, 14 watt air pump running 4 air stones, and light is 400w HPS 30 inches above canopy.

That light green color is often caused by low pH or N deficiency. If you're certain that your pH reading is accurate, I'd give them a healthy dose of high-N fertilizer.

The slight damage on the leaf blade edges is probably K def but since it's already mixed with another problem, it's hard to be sure.

Cool thanks finest. I've upped the PPM to 550 by adding some Florvanova grow on you and a friends suggestion. I'll let you know what happens.