Wonder why my ph is declining??

[picture the reef]

I have a 90 gallon reef tank with a few sps and a few fish. I am running a calcium reactor with an effluent ph of 6.6. I have recently added a kalk reactor which is hooked to a Litermeter dosing pump that replaces about 6 liters of evaporated water a day. My alk level is 9.3 dKH, calcium is 420 and magnesium is 1290. My ph drops to about 7.90 at night and never gets above 8.10 during the light cycle. The strange thing is that the ph will slowly rise for the first 5 to 6 hours of the light cycle and then will begin to decline the last 4 hours to a point of less than 8.0 about the time the lights go off. I have tried increasing the stir frequency and duration of the kalk reactor. I'm now at 25 minutes every 4 hours up from 15 minutes every 6 hours. I would like to get my ph a little higher during the day but what I really don't understand is the drop of ph during the light cycle. Thanks for reading this long and probably confusing post. I would appreciate any input.:)

[mattboy]

I'm sure you'll get some other posts on this, but my guess would be that you might have an oxygen deficiency; maybe there's alot of CO2 in your room air, or you're not getting enough gas exchange. you might try running a skimmer for awhile; that will make sure your water is O2 saturated, assuming there's enough in your room air. Some people with well sealed houses and lots of people living in them have run skimmers with the air input vented to the outside. Try taking a sample of your water, running an airstone in it for several hours, and check the ph; if it goes up significantly, there's your problem. OTOH, I wouldn't consider a ph fluctuation of 7.9 to 8.1 a problem.

Matt

[Randy Holmes-Farley]

I agree that CO2 could be the issue (not a lack of O2).

For example, does the pH drop when more people come home?

When cooking with gas begins?

When the windows get closed for the night?

Anything else happen in the system at about that time (like a refugium light going off)?

[picture the reef]

I'm running an Eco-Wheel filtration system so the I am not running a true skimmer as such. I am running a large air pump that powers the system but it has no provision for outside air input. Randy, we do not cook with gas, however this is about the time everyone returns home. The tank is located in a fairly small room but I do keep a ceiling fan running all the time primarily for heat removal from the metal halides but I am sure this helps with the disspertion of CO2.

[Randy Holmes-Farley]

but I am sure this helps with the disspertion of CO2.


Yes, but if the room is elevated in CO2, as they often are, then blowing it around doesn't help.

Here's an article that describes indoor CO2 issues:

http://www.advancedaquarist.com/issues/apr2002/short.htm