On Feb 2, 11:51 am, NitroCarbon <[email protected]> wrote: > On 02/02/2011 01:28 PM, NitroCarbon wrote: > > > 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. > > BTW, unbuffered water fresh out of the RO unit has a pH of 6.4 to 6.8.
Gah. The problem is the Acid Buffer. I've never used it in a heavily planted tank because I always add only carbonate and CO2. I was using Acid Buffer to lower pH in with my "liquid rock" tap water and now I see why it worked so nicely. >From Seachem's website "As Acid Buffer™ lowers pH it converts carbonate alkalinity (KH) into available CO2. It may also be used to safely lower pH in marine water." >From the MSDS, Acid Buffer is apparently bisulfate salts, probably sodium and potassium. Sulfate has a very low pKa so it's acting more as an acid than as a buffer. "Buffer" is a complete misnomer. So, what's happening is that you add Alkaline Buffer/Acid Buffer. You get the pH Seachem has determined in your RO water and the water has a fair amount of CO2. You add the water to the tank, plants use up the CO2, the acid buffer starts eating at the KH. Sulfate doesn't buffer effectively at aquarium pH ranges so what's left of the carbonate system takes over and pH climbs as plants happily strip all the CO2 from the water. You end up at the natural, high pH for water with very little buffering capacity and very low CO2. Simple solution: Stop using the Acid Buffer. After digging about on the Seachem site, I'm sorry I even suggested it for RO water. Ugh. You're just chasing your tail, lowering pH by releasing CO2 and breaking the carbonate buffering system. I thought that product was something sensible like ascorbate, not bisulfate. Until you get CO2, set KH to 2-3 with Seachem alkaline buffer, put GH where you want it. Keep your tank well aerated to keep the CO2 concentration from bouncing from photosynthesis. It will probably still go up by evening if the tank is brightly lit, but there isn't much to do for that. Your fish should be fine if the water is clean and soft, even with the pH a little high until you get CO2. Do keep an eye on KH to be sure your plants don't "eat" the carbonates too fast between water changes. You may have to add a bit of alkaline buffer every week. For non-CO2 tanks, many discus keepers use blackwater extract or peat to soften and condition the water and lower the pH slightly. --Altum
