HELP! RO ph is 5.9

I have a purple clone in its second week flowering. It is a very small plant and is in a 1 gallon pot. when i turned on my lights i saw my lady looking a little sad. The newer growth is curling down and curling on the sides. I decided to check my feeding water to check make sure the ph was right. it was 7.1 woops so i dcide to flush and flush with 6.6 water and am getting a RO of 5.9 How is this possible? I flushed with 2.5 gallons and the RO still has not gone up at all. Used Fox Farms Grow big only so far and not even full strength.

when is the last time you calibrated your ph meter?

I have only had it for about 2 months and they guy calibrated it for me when i got it. And I know the ph of my bathroom sink water is always 8.1 so i use that to make sure its still calibrated.

Ph buffering is your friend with soil. I use a special homemade soil blend, chalk full of perlite, humus and some dolomite. With these, in right proportions, you can keep a rather stable ph in your soil, even after the acidic affects of nutrients and composts. I take it RO= run off? (RO is traditionally used for Reverse Osmosis is why I ask). You might want to add more dolomite to your mix to make the soil slightly more alkaline going forward if you are getting runoff at 6.6.

To fix your problem, I would recommend a light (and well mixed) application of sodium carbonate (soda ash) to your water to raise the ph slightly. A little of this goes a long way, but its very effective and harmless is small doses. Sodium bicarbonate (baking soda) can also be used if you are in a pinch, but it is not as efficient.

Flushing should be reserved for the extreme cases of imbalance or issues. 7.1 is not extreme for soil.