Any help would be great

[mari-ju-ana]

I have a home made wick and reservoir hydroponic system I have 6 plants in a reservoir approx 8 gallons of water. they are growing like crazy..... healthy I have a 400w hps light and hood. been doing a water change approx every 7 days and we are basing that on the Ph reading... we are adding floragrow (2-1-6) I have air raiders in the water and I cannot seem to stabilize the PH. I am treating the water aggressively with the nutrients (3 tsp. per gallon as per the bottle) I changed the water Sunday, with a tested PH of 6.8, today is Wed and the PH has lowered out of our 6.3-6.8 range...... It dropped to approx 5.8. What can I do? Is this the plants using up the nutrients that quickly? Not enough room for the plants? We are adding just about 1/2 gallons of water (treated with the floragrow) daily to keep the level consistent. Any help would be appreciated...... and should we mist the plants? the humidity is excellent so we are not misting. Just enjoying the grow......