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pH Fluctuations

Damien Buckley

28 Sep 2013
Greetings from Brisbane (Australia). I've just come back into the hobby following a fairly extended hiatus and have set up a Fluval Edge 46l with the following equipment:

Stock filter & lighting (LED strip, Aquaclear hang-on)
Eheim Jager Heater
Fluval 88g CO2 kit with Dennerle shut-off valve

The tank has been running for 3 weeks and is looking good thus far - ammonia, nitrite & nitrate all 0, 6dKH. Water temp 25 degrees (can rise to 27 depending on ambient temp…).

I'm dosing approx. 1 bubble CO2 per second, starting a couple of hours before lights on and shut-off a couple of hours before lights off. Tank is getting 10 hours light a day - the Fluval LED's look nice but are a bit weak.

pH seems to be 7.4 on a morning before Co2 / lights dropping to 6.8 by the evening. I have a Sera Co2 indicator which is showing green but having read other posts here I need to get hold of some 4dKH solution - its currently filled with tap water.

My question is, are these kinds of daily pH fluctuations acceptable or should I does CO2 24 hours instead? I'm assuming that the overnight drop is due to the hang-on filter causing CO2 loss.

Things have changed a fair but since my first Dennerle tank a couple of decades ago and I saw some mention here that I should not worry so much about indicated pH values which is entirely contrary to when I was in the hobby before so just thought I'd throw it out there for some opinions.

Fish and plants all seem pretty happy. 6 x Black Phantoms, 3 Corydoras Julii and a couple of Ottocinclus plus a nice crew of cherry red shrimp that I was lucky to inherit with the plants. I'm planning on finishing the stock off with a pair of Rams but want to ensure that the tank is fully matured and stable first.


REALLY nice forum by the way. I'm a web-developer by trade and rarely see forums look as good as this one.

Marcel G

Hi Damien,

pH fluctuations in the range of 0.5 to 1.0 are quite normal if you supply CO2 only during the photoperiod, and shut it down overnight. Some time ago I have asked the same question as you did, as I have read some articles which stated that pH fluctuations are bad for fish. Still I was never able to find out any scientific article which would confirm this statement. So I believe that the pH fluctuation could be dangerous to fish, if it falls outside the range of their tolerance. Each fish has some pH range. For example Otocinclus needs a water with pH range between 5.5 and 7.5 ( Otocinclus macrospilus – Oto — Seriously Fish). So if pH falls outside this range, then it could be dangerous for this kind of fish ... and similarly with other fish. But pH fluctuations which occur in this range should be safe. Also realize that the pH fluctuations in our tanks are relatively small (in the range of 0.5 to 1.0 point). So if the pH drop would be 3-5 points per day, that could be bad, but pH drop of 0.5 should be OK. This seems to confirm the fish themselves, as many fish reproduce without any problems in our tanks despite these small pH fluctuations.

Still if you think this relatively small pH drop could be harmful to your fish, you can supply CO2 non-stop (24/7), BUT I would say that constant higher CO2 concentration could be worse for fish then regular (yet small) pH fluctuations. If you add CO2 non-stop to your tank, the fish will be exposed to the higher amounts of CO2 constantly without any chance to have a break (e.g. overnigh). So in the case of non-stop CO2 supply I would recommend to stay at 10-15 mg/L CO2, otherwise your fish could experience 24/7 stress.



Expert/Global Moderator
11 Jul 2007
Chicago, USA
Fish do not care about pH or about pH fluctuations. It doesn't matter what the range is.

What we knew a few decades ago was wrong.
What we know now is less wrong.
Who knows what we will know tomorrow?

The pH value measured in a tank is a mirror showing a reflection of some condition in the tank. The value can therefore reflect a wide range of events. Some of these events are harmful, and many of these events are negligible. It is necessary therefore to understand the reasons for the pH change or pH fluctuations. Then this determines whether it is bad for fish or whether it has a negligible effect.

pH must also be understood within the context of other parameters so that it's effects can be compared with or discussed in terms of other tanks or other situations.

a pH drop of 1 unit means an acidity increase of 10X, That is actually a huge swing, but again, if it is due to CO2 then this is negligible.

Making a decision to inject CO2 24/7 just because you want to maintain a more constant pH is probably the worst reason imaginable for injecting CO2 on a 24/7 basis. There are some very good reasons for 24/7 injection, but this is definitely not one of those reasons.

Use your pH measurement as a tool for reading the behavior of and for optimizing tank health, not as a witch hunter to persecute absolute values.