You are going to struggle a bit, but you can bring the pH down with white vinegar as well. Like lemon juice, it's not a strong acid, sulphuric acid would be better, but that's more dangerous. Leave it alone if you're not familiar with working with strong acids.

Assuming you are using horticultural grade rockwool, it should not be throwing off your pH by very much.

I always rinse my rockwool off in plain [tap] water before I put it into a systme but I have never experienced dramatic pH shifts from using rockwool.

The quick and easy solution would be to try rinsing the rockwool, if you have not already, by running plain water through it and allow it to drain completely before reconnecting the nutrient supply.

If you still can't get the pH under control, I sugest purchasing real hydroponic pH adjustment chemicals instead of raiding the kitchen for DIY acids and alkalis.

Good luck.

Just curious: What are you attempting to grow?

Before you go nuts with all this, check out several nutrient charts to see what the proper EC and pH levels are for what you're growing. In many cases, the range is such that it's not all that important to be very 'exacting' with the numbers.

Browse through the forum links. Other members as well as I have put up various charts from various sources. Key words: "nutrients" "EC" "pH" "chart"