CO2 vs. low kH...a losing battle???

[fishluvr]

I seem to have a small problem...

I'm running a pressurized CO2 system on a 120 gal tank and my pH refuses to drop below 7.2 (according to my Milwaukee SMS122 controller). As soon as the lights come on, my pH begins to rise at a rate of about 0.1 per hour, until it gets back up to about 7.6 (where it was before I added the CO2 on Saturday).

I'm running 3.2 wpg (384 watts) for 5 hours, off 2 hours, then on another 6. My CO2 is set to bubble about every 2 secs now (started at every 6 secs)...

I've tried several test kits to verify my data, but so far, it's looking like there may be something wrong with my controller.

These are from a control sample of my tank...
Kit 1 shows: pH 6.8*, Nitrate <10ppm, Nitrite 0ppm, gH 75ppm, kH ~40ppm

Other tests show: pH 7.0 , kH ~2dkH

SMS122: after calibration (with 7 and 4 solutions) showed pH 7.6

*Kit 1 has registered pH at 6.8 on every test, incliding with the bottle of 7.01 I've got, so I don't trust that reading...

Does anyone have any idea what could be holding the pH at 7.2? I will be trying to find out what my local water conditions are from the local sanitary commission...

 

[_Nemo_]

You need to bring your dkH to about 6 IMO. You can use plain baking soda to do it but that will bring up your pH. You have to dose then a lot more to bring the pH down to where you want it to be - but it will be stable at that level then.

Keep notes of what you have added - it is a trial and error exercise. Hope you have no fish in this tank .

 

[betowess]

Yeah, at 2 dKH you may see wide PH shifts. I agree you need more KH buffer and the CO2 ppm will come up and the PH should drop. I think. As you probably know its normal for PH to rise when the sun is out. Good luck.

 

[Guest]

You should raise the kH a bit. But with the controller it's not as critical. And if your controller is calibrated then I would trust it over the test kits. And for a 120 gallon tank your CO2 injection rate sounds very, very low. I would start by increasing the bubble rate to at least 100 bpm. You have the controller you don't have to worry about getting too much CO2 into the tank. In fact to get decent CO2 levels you would have to get your pH down to 6.4 with your current kH. So you might want to increase your kH.

 

[fishluvr]

Thanks, all..Right after posting I started thinking about going with the inreased CO2 flow due to the tank size, and just letting the controller take it from there, but I didn't really trust my instinct on it...

Since I do have some fish in there, I set the controller at 7.4 and turned the bubble rate to about 70 bpm...the tank seems to have stabilized, and I've slightly increased the bubble rate again (to just over 100 bpm...seems to be much closer to what I need...thanks, Rex) and I've lowered the setting to 7.3...I plan on continuing this 0.1 pH change every couple of days until I get down to about 6.8 or so...

Thanks again to all...