CO2 and PH levels


Junior Poster
Mar 13, 2009
Hi everyone,
I've been reading this forum for a while now and can say that this is probably the best resource for planted tanks on the net. I'm slowly trying to absorb all the information so I could make my tank flourish and with the help of this forum I’m moving to the right direction.

Now here is my question. I'm trying to keep my CO2 levels as high as possible without harming fish and my Co2 is connected to timer so it is turned on 1hour before lights. I've found out that in the morning ph is 7.2 where in the evening it drops to 6.6 and some fish does not look happy about that.
Is this normal (I feel not)?
Should I consider buying PH controller and connect my CO2 to it and run it 24/7 or is there other way to control ph?

Here is tank info:
4x54W T5 tubes 8 hours
2KG CO2 with Rhinox5000 (8 hours)
2 Filters
KH 8
GH 14


Lifetime Charter Member
Lifetime Member
Mar 12, 2009
What're you keeping in terms of fauna? What isn't looking happy?

You could try retaining more CO2 by decreasing surface disturbance, which may require you to reduce your CO2 output a bit. A couple short night time bursts programed on the timer might keep a more even pH level; not sure what it'd do to the algae though.

I experience a swing of up to 6.15-6.7 pH every day. It hasn't hurt anything so far; tank stock includes pencil fish, otos, yamato shrimp and apistos.



Lifetime Charter Member
Lifetime Member
Jan 15, 2009
You might be running a bit too much. Can you describe the "not looking too happy" a bit more? The easiest thing to do would be to just drop the amount going in just a touch and then see where things stand at the end of the day. If the fish are happy at that point I'd probably tweak it just a touch more until you are as high as you're comfortable with and the fish are acting normally throughout the day. Generally speaking, the pH controller will not get you much in this application and may make things worse as the rates of CO2 going into the water will be more random. pH controllers only seem to make sense as either a failsafe or in marine tanks to control a calcium reactor.



Guru Class Expert
Jun 8, 2007
It sounds like your pH is just swinging with the fluctuating levels of co2. I don't think fish usually mind this type of fluctuation, as it doesn't affect KH, GH or TDS which are more important to fish than the actual pH level (which is usually indicative of these things, but that relationship falls apart once you start injecting co2). If you are really concerned about your pH fluctuating, leave your co2 on 24/7. If your fish seem stressed, I'm thinking that the reason is probably not that the pH is changing, but something else. It could be co2 getting too high, do you have a drop checker? I think your pH is gradually lowering probably because you don't have enough co2 in the tank to start with and it's gradually building up over the course of the day. Either that or you start off with enough but then end up adding too much. A drop checker would let you know in either case how your co2 is and then you could move on from there to other issues. For the sake of your plants you want to keep co2 stable throughout the photoperiod, and for the sake of your fish you don't want to be adding too much co2. A stable, moderate level is what you should be aiming for, which will also give you a stable pH throughout the day unless something else strange is happening in the tank.


Guru Class Expert
Feb 17, 2007
Lincoln, UK
Your fluctuations are pretty similar to mine except mine are 7.2 - 6.4 ;)

As pointed above people talk about PH fluctuations affecting fish as it is easier to understand and test for than the real problem which is KH.

KH Crash is what Ph crash really is.

Ph fluctuations due to CO2 injection however do not affect KH and therfore shouldn't affect the fish either. More than likely the CO2 is too high or in certain areas etc.

I have a pretty good surface turbulence 24/7 so the minute the CO2 turns off the Ph will return to 'normal' much faster than in a tank with minimal surface turbulence and the fish are fine with that.



Junior Poster
Mar 13, 2009
Thanks a lot for you replays.
Now it looks to me that the problem is caused by CO2 not PH.
The stressed behavior is that the fish are not active, they simply stop swimming. They are not at the top of the surface but do not appear normal to me.
I have a drop checker but I think the reagent is not good. One day I had too much CO2 in my tank but it never became yellow, even though my fish were ready to climb of the tank.
It’s a cheap no name drop cheker that I’ve got from eBay. It tells to put tank water and 3-4 drops to reagent? Also I have to change it every 48 hours because fungus starts growing on it. I’ve ordered different reagent and will put it back as soon as it arrives.
I’ve lowered co2 and see what happens next.
Thank you again for your help.


Prolific Poster
Nov 7, 2007
SF Bay Area
Looks like you have already gotten lots of good advice. FWIW....My fish also become inactive from too much CO2. For a long time I thought they just preferred hiding in among the plants but if I drop the CO2 levels they start to swim around the tank again.

Your drop checker is not giving you an accurate reading because you are using tank water in it. The directions that come with a lot of the drop checkers tends to be wrong you don't want to use tank water. You need to use 4 dKH water along with the reagent you are given. The reagent is bromothymol blue, same as used in the standard pH test kits. You can make your own 4 dKH water (do a search on this site for directions) or buy lab grade. See link below...

KH Standard



Lifetime Charter Member
Lifetime Member
Jan 24, 2005
Sacramento, CA
Running the CO2 24 hours a day, with a constant bubble rate, doesn't give you constant ppm of CO2 in the water, nor constant pH. This is because the plants will be consuming a sizable fraction of the CO2 during the lights on time, but none at all during the lights off time. So, that constant bubble rate will result in a higher ppm of CO2 at night, than during the day. This will not be appreciated by the fish, and, if you run the maximum your fish tolerate during the day, the fish will very likely not survive the night.


Junior Poster
Mar 13, 2009
Just a quick question to clear my mind regarding CO2.
Do plants need CO2 during day time when the lights are off?
The tank is located in the room facing southeast.The tank does not get direct sunlight but it is quiet bright in the room since early morning and my lights are only on from 1PM.

Tom Barr

Staff member
Jan 23, 2005
Some fish are less active during the day, but behavior can be a funny thing.
I look at breathing rate, color etc, mostly the larger fish, or the very active, hyper active fish. Shrimp also behave differently.

You can also not add CO2 for a day and add Excel instead and watch the behavior.
Seems like there's enough CO2, circulation is what is left. Make sure there's good circulation and enough water movement on the surface! Enough soi that you do not break the surface of the water, but so that there's some slight ripple in some parts.

Respiration is a two edged sword.
You need some circulation to get O2 into the system, you will lose a little CO2, but adding more is not an issue.

So adding good CO2 is both a CO2 rate(bubble rate) and an O2 rate(via water movement and circulation)

Tom Barr


Junior Poster
Sep 3, 2007
Nos;35615 said:
... I'm trying to keep my CO2 levels as high as possible without harming fish and my Co2 is connected to timer so it is turned on 1hour before lights. I've found out that in the morning ph is 7.2 where in the evening it drops to 6.6 and some fish does not look happy about that.


I am not sure if you are getting 6.4 before or after the lights go out. If it is before the lights go out, then its pretty simple - you are injecting CO2 at a faster rate than the plants take it up, so just turn down the rate. If it's after the lights go out, then the picture is more complicated.

I've been injecting CO2 now for about a year. I was quite lucky in that a friend of mine works in industrial water treatment and his work was throwing away some sensitive (± 0.02) pH probes and a controller.

All of this is now hooked to a computer which gives me a graph of the pH over the day. I know you don't really NEED any of this to inject CO2 but it was free and suits my personality. What you describe with pH is what I have observed in one form or another ever since I have been injecting.

My fish don't care about low, low pH (6.2) because the kH is only 2. Although it has been strongly criticised, I still use the KH/pH tables as a guide for how much CO2 is in the tank. It has been noted that CO2 varies widely within the tank anyway, so any measurement system is not going to be accurate to the last ppm.

In any closed system where you are trying to manipulate a variable, you will have overshoot and undershoot, whether you are injecting by timing or by a controller. It is more complicated when you add a 2nd variable into the equation - fluctuating light. Remember that when the lights go out, your plants switch from photosynthesis to respiration. That is, they start using O2 and making CO2. I am not sure how much CO2 this will actually add to your tank, but at least some.

Anyway, here is an experiment you could do for yourself. Don't inject any CO2 for a day or 2. Then measure the pH throughout the day. I think that you will find it swings quite a bit anyway, and is lowest just before the lights come on. How low you can't know until you measure it.

The bottom line is that the more CO2 you inject, the lower this low value will go. You Could try injecting less, or maybe shutting it off sometime before lights out.

I might be telling you a lot you already know, but then again, maybe not.



Junior Poster
Mar 13, 2009
Thanks for replay.
My Co2 tank ran out just before Easter so I had good opportunity to monitor ph levels.
They do swing during day but not much 7.3-7.4 according to the tests.