I can't make sense of any of it. Water at KH6 in atmospheric equilibrium just off an RO unit should be somewhere around pH 7.8-7.9. Atmospheric CO2 is around 2 ppm. If the pH is substantially lower, either a test kit is wrong, there is an odd CO2 buildup in your RO water, or there is an acid other than carbonic acid in the RO water. You mention buffers. I hope you are not using a "discus" phosphate buffer. You will get algae blooms from fertilizer balance problems in a planted tank.
Your goal of KH6, pH7 requires either CO2 injection or too much phosphate buffer, so you will have to settle for a higher pH for now. Out of curiosity, what made you choose the oddly high goal of KH6 for discus anyway? I wasn't kidding when I recommended KH4 in another thread. There is no reason to add all that buffer when you want pH below 7.0 anyway. It is not surprising at all that brightly-lit plants are acting as a carbon sink. Photosynthesis can also raise pH as plants remove CO2 from the water, but I'm surprised there is a big shift with good surface agitation. In a tank with still surface water, it's easy to get pH shifts of 0.5 or larger from morning to evening as plants use up CO2. Fish and plant respiration replaces the CO2 overnight and pH will fall again by morning. The plants may still be driving some pH shift in your tank if they are using CO2 faster than it diffuses in. If there is as much CO2 in your RO water as the nonsensical pH measurement suggests (though I have no idea what 18 ppm of CO2 would be doing there!!!) the plants would pull pH within 4-6 hours. Then they will start in on the carbonate buffer. My first reaction would be to get fresh test kits. Also, agitate your RO water well before you measure. As I said, unless you are adding phosphate, RO water at KH6 should not have such a low pH. --Altum
