how much to increase nutes with co2?

Hey guys, I have some ak 47 in an Ebb and Grow, using GH 3 part solution.

RUnning about 850 ppms, now starting second week of flower, with co2 at 1500, in a 5x10 space,with 2 (1000) hps.

They are growing well, but leaves, loook a bit pale,, should i increase the ppms qutie a bit? How much more nutes do plants take up when co2 is addded?

I haven't used GH in over a decade, but if I remember correctly I was able to go up to about 1,000ppm without burning. Maybe someone who still uses GH can verify this for us....

up to 2000 in ebb & grow
also do maybe 4 cycles per day....or 3...