Overfert with only 700ppm

I have 6 plants that are going to be mother. I use tap water which has 150ppm. I add ferts which increases the ppm to around 700. the plants are about 1 1/2 months old and about 1.5 to 2 feet tall. They all show over fert signs with burnt tip (old and new growth). I guess i should lower the ppm but i am looking for any help. i run a airstone in 5 gal buckets with 1000watt mh.

what's your pH? what are your ferts?
ppm is important in the correct pH range, you can't live by ppm alone.