On 02/02/2011 12:36 PM, Altum wrote: > 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.
No no. Water off the RO unit has KH of zero. It only gets up to 6 when I have buffered it. After buffering I have a KH of 6 and a pH of 6.8 to 7.0. > 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. No, I'm not using a "Discus" buffer. I'm using seachem acid buffer + seachem alkaline buffer at a ratio of 1:2 (acid:alkaline) as suggested by seachem in order to obtain a pH of 7. It so happens that I get a KH of 6 and a pH of 7 by using this combination, at proper levels per gallon as directed on the labeling for each buffering compound. For curiosity's sake I once experimented with baking soda (calcium carbonate) as a stand-in for the alkaline buffer (only in test water that never went into the tank) and I came out with a pH that is higher. I have decided to stick with what is recommended on the labeling for the buffer compounds. Also, I agitate and mix the water very well both when adding the buffers and 3 days later when testing the buffered water. Consistently results in water with pH 7 and KH 6. The same test kit tests my tank water at pH 7.6-7.8 and KH of 2 when measured 24 hours after a water change with the source water. I too cannot explain this extremely unexpected and very curious water situation. I'm baffled. > 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. I could still go for the stated 6.8 pH and KH of 4, but I'm wanting to get there gradually. So far, as all must have noted by now, progression toward that goal hasn't even remotely begun to happen. > 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. Me too! > My first reaction would be to get fresh test kits. I bought my test kit about two months ago... but I'm going to have to buy another. ~nc
