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Re: PH crashing up?

C. Alan wrote:

I am cutting the tap water with RO water to reduce the amount of Nitrate in
my water.  I live
in an area with a lot of farming around it, and in the summer time when the
water tables get low
the nitrates can get has high as 50ppm.  Right now, my nitrates out of the
tap run about 20ppm.
The tap water runs about 3 degrees of GH and KH.  I basicly use the tap
water to stretch my RO
water supply.

My bad. I had it reversed. I should really quit posting at the crack of dawn after little sleep -- I also failed to notice where you say the KH in your tanks is 2 or less, which I thought was higher for some reason. It does seem strange that pH should go so high, but after reading his reply to your post, I think Larry Jones may be on the right track -- the plants could be taking carbon dioxide from the bicarbonate in a process called biogenic decalcification. Some plants are very good at it and can drive the pH surprisingly high, particularly in a small container. Another tell tale sign is a white powdery substance called marl (it's calcium carbonate) which develops on the leaves of some hardwater plants. I sometimes find it on certain plants in my tanks if I let the CO2 supply dwindle. Two 40W bulbs put quite a bit of light into a 10gal and no doubt CO2 is in short supply, and those bubble-up sponge filters are also driving off CO2, so it wouldn't surprise me at all if the plants were behind the mystery. However, the Amazon swords you mentioned aren't capable of biogenic decalcification. I'm not sure about Java Moss. The floating mats of algae are the most likely candidates among the three you list in your original post. If you have access to it, Diana Walstad's book, _Ecology of the Planted Aquarium_, has some good info on this process as well as a short list of plants that are and aren't capable of using bicarbonate.

If biogenic decalcification is at the root of the high pH, you can stop it by supplying CO2 to the plants through one of several means, or you can reduce the impact on the KH and pH by raising the lights some to slow the rate of photosynthesis. Balancing a planted tank can sometimes be a tricky and convoluted business.
Chuck Huffine
Knoxville, Tennessee