Alkalinity does not drop...why?

[Ostri]

My alkalinity has been quite stable at around 8.4 while I've unplugged my KH dosing line for more than 3 weeks. My ph drops to 7.8 at night sometimes because of high CO2. Its a 220g acropora tank with many corals and 3 clams. I don't quite understand how alkalinity can hold up without dropping. It used to drop like crazy. Is it because of CO2 concentration? Anyone else experiencing this in the winter time?

Thanks.

[bertoni]

If thing is being dosed for alkalinity, the zero consumption indicates very little growth in terms of skeletons, and thus most likely little growth of any stony corals or coralline. Tap water and pH buffers both can add a lot of alkalinity. All pH buffers do so, despite any advertising.

[Ostri]

Originally Posted by bertoni

If thing is being dosed for alkalinity, the zero consumption indicates very little growth in terms of skeletons, and thus most likely little growth of any stony corals or coralline. Tap water and pH buffers both can add a lot of alkalinity. All pH buffers do so, despite any advertising.

Thank you. I do add a lot of RO/DI water. But my rasin's been in there for a while. That could be why... My coraline is doing very well.

Edit:

I just put in some RO/DI water into the system and I did an alk test before/after that and found that Alk increased by 0.3 (8.4 to 8.7) after addition of RO/DI water. Does that mean that I need to change my carbon filter or DI beads?

[bertoni]

I suspect that a small difference like that is due to measurement error, but it's possible that the DI needs to be replaced. You could run an alkalinity test on the RO/DI. The result won't be all that accurate in fresh water, but it'll give you a general idea of the level.

[Ostri]

Originally Posted by bertoni

I suspect that a small difference like that is due to measurement error, but it's possible that the DI needs to be replaced. You could run an alkalinity test on the RO/DI. The result won't be all that accurate in fresh water, but it'll give you a general idea of the level.

I noticed that at night the pH drops to 7.8. I'm not sure if it's related or not but overnight I can many corals have burnt tips and Alk is pretty high. That makes me wonder if the decreased pH caused some reactions on the rock or something to increase alkalinity.

[bertoni]

pH at 7.8 is high enough to be safe. Lots of tanks of the month have run at pH 7.8. I might get a second opinion on the test kit, since we see a fair number of testing problems, though.