PH woes

[munkyface]

OK, so I have this really sweet thing going here, but I am having an issue that I can't seem to get under control. I have a 3 gallon reservior growing 4 babies in rock wool under a T5 Sun blaze light. The problem is that I have consistently high PH. My tap water tests at about 7.5. I can adjust it to a working range (5.8) but as soon as I add it to the reservoir the ph jumps up to about 8.0. I have tried commercial PH down and 2 types of mild acids, but I still get the same results.
I understand that proper PH allows for greater nutrient uptake, so I am starting to get a little worried. The babies are beginning to look a little yellow. Any help?

 

[rb2006]

Welcome to MP! What kind of nutrients are you useing?

 

[munkyface]

Fox Farm Grow Big Liquid 3-2-6. 2.5 tsp in 3 gallons. Last (and only, thus far) nutrient addition was on 9/3. Lights are about 3 inches from plants. Ambient room temp is 79.3. Temp under lights is 80.4.
I was reading on another thread about using distilled water, but I dont think that will help the PH issue. Can the rockwool or clay rocks that I am using make a difference?