PH constantly fluctuating...

squishyfishy Member Posts: 10
Ever since "reviving" my aquarium earlier in the year, the PH has always fluctuating. The highest being 7.5PH and recently the lowest at 6.7PH. I've been noticing a pattern however...
During the day it would decrease over time by ~0.2PH and then climb up again over night by roughly the same amount but sometimes even more.. or less.

Tank Parameters:
  • Tank: 200 litres
  • PH: 7.3 (although less than a week ago it was going between 6.7 - 7.0)
  • KH: 5 (89ppm) - slowly attempting to raise it some more with KH powder +7.0 (I have been doing so for a while now)
  • GH: 5 (89ppm)
  • Nitrates: ~40
  • Ammonia/Nitrites: 0
I've also been changing water every 1-2 weeks to help manage the nitrate as the early stages of the "revival" were showing high levels of nitrate (at that point I only had guppies which are apparrently immortal).

Tank inhabitants:
  • Many Guppies
  • 2x Dwarf gouramis
  • 1x Sucking catfish
  • 15x Cardinal tetras
  • Various types of snails
  • Plants - Water sprite lace fern / Anubias / Java moss / Hair grass (overall, not heavily planted)
  • Rocks (tests seem to show that it doesn't alter PH)
  • Gravel (tests seem to show that it doesn't alter PH)
  • Driftwood - medium size x2
  • CO2 - Injecting through diffuser via bottle setup.
CO2 is on an automatic timer. Turns on after the lights come on and turns off a couple of hours before lights go off.
After some observation, it seems that the CO2 injection is keeping the PH from climbing way to high also and am worried that if I stop it, it won't stop climbing.
Tests confirm CO2 levels are safe.

Would this degree of fluctuation be stressing out the fish?
How long is the KH adjustment period after adding KH buffer?

I've done quite a bit of research on the matter but can't seem to figure out what might be causing this.
Ideally I would like to have it steady around 7.1PH without minimal fluctuation if possible (unless it's a natural thing considering my setup).

Also, any advise in achieving a better balance would be greatly appreciated.


  • Bob
    Bob Member, Classifieds Posts: 708
    I remember reading somewhere that ph fluctuates with CO2 concentration.

    During darkness plants use oxygen and produce CO2 which results in higher concentration of CO2, I cannot recall if ph drops or rises as a consequence but I will check to see if I can find a link for you.

    Here is one,


  • squishyfishy
    squishyfishy Member Posts: 10
    Thanks for that, nice article.

    Maybe if I increase the duration of the CO2 injection, I could lessen the fluctuation rate and partially save my kH from being consumed by the plants as there'll be more co2 present? I'll give it a go.
  • Frayk
    Frayk Member Posts: 1,009
    Personally, i'd stop dosing with KH powder, it would seem that its not working, secondly do you really need CO2?
    Ph fluctuations wont hurt your fish, the .5 up or down, are minor.
    Just my opinion, perhaps sort the water Hardness issue out prior to putting it in your tank.Cheers.