I've had some fish deaths lately and paying attention to water params, I've gotten a little concerned about my pH levels. My tap water comes out of the tap with a pH of around 7.6. After aging it overnight it ends up being around 8.0.

However, a week after a water change my aquarium tends to be around 6.8 - 7.0. If I then do, say, a 50 % water change it puts my pH up to around 7.2 immediately and then around 7.8 the next day as it ages, which can give me a swing of up to 1. This seems like a lot for the fish! And then from 7.8 a week later I'm back down to 6.8 - 7.0.

The tank is planted and CO2 injected but the CO2 is on very low. My KH and GH out of the tap are both around 5 drops, but after a week in the tank this also drops (down to 2 - 3 drops).

I'm trying to figure out a water change schedule or something else I can do to keep things more stable and I'm starting to wonder if the only solution is to do water changes every 2-3 days, which seems like a real hassle!

  • 1
    How many litres is the tank? A smaller tank is harder to keep consistent. – Henders Apr 23 '19 at 8:35
  • Are you taking PH measurements at the same time of the day under the same conditions? – Jestep Apr 25 '19 at 18:05
  • it sounds to me like your water supplier uses CO2 to adjust the PH down to neutral and when the CO2 have been released from the water you end up with alkaline water. – trond hansen May 23 '19 at 15:41

A while ago, I started to drink bought water, marketed as having a pH of 8.0 - to compensate (according to my guessing) for all the aggressive acidic food and drinks that we eat. Result: in just 2-3 days, my throat became irritated and started coughing and so on. I cured this by no longer drinking that water.

So it is definitely not good to add that water in a tank. See this article about the better way to test the tap water. Basic idea: put the water in a bucket for at last 24 hours, better 48 hours, with an aerator - to break the surface of the water and improve gas exchanges. Measure whatever you want after that.

I had a bowl with fish, less than 15 liter capacity - very small by most standards. However, the gravel on the bottom was full of decomposing matter, as well as the mechanical filter. Additionally, I had some basic stones (similar to those under train tracks, properly washed before use), some aquarium wood, and fishes. The only things I needed to do were:

  • feed the fish once a day;
  • add some water when evaporation lowered the level visibly;
  • wash the sponge of the filter when there was (almost) no flow possible through it - only mechanical cleaning, no chemicals; I added an extra layer of dense cloth to make it ale to catch finer dirt particles.

The catch was that the water was plain bottled water, not tap. I do not trust the tap water for any health-related purpose.

The longer lived fishes lived in excess of 4 years - I never took notes on their life-spans, no birthday anniversaries...

So a good balance can be reached even in the so dreaded small tanks.

Bottom line: try to find another source(s) of water, at least for a while, until you understand the root cause of the issue.


A 50% change is quite big and might not allow much "stability". In my opinion it's too much and indeed will be followed by a period of "swinging" of many parameters and bacterias. Everytime you start with pure tap water, which becomes alive aquarium water over time, until half of that "work" is negated at the next water change and the stable parameters of the established tank are disrupted. I would suggest 25%, the spikes in pH and other parameters will be more smooth if the volume changed is smaller but more frequent (as you already guessed). It doesn't need to be every 2-3 days. Monitor the parameters, once a week might be ok on an established tank.

I have read that aquarium water tends to become a little bit more acidic over time but I don't remember why. It has to do with decomposing matter, so that would be normal. I don't know why your tap water goes from 7.6 to 8.0 overnight, it might be normal but 8 is indeed a bit high for aquarium.

By the way, ammonia acts as a basic substance: it wants to capture hydrogen cation to go from NH3 to NH4+ and become less toxic. If you pH is already high, the water itself competes with ammonia for the hydrogen cations, so it stays toxic. If the water is a bit more acidic (giving off some H+) ammonia will be more easily converted into ammonium. That's not related to your problem but just something to keep in mind when you measure pH and ammonia. As you already know, stability is by far the most important thing with pH, compare it with altitude for humans: it's OK as long as it doesn't change too fast.

What kind of fishes are in there? And plants and filtration?

  • "A 50% change is quite big" it depends, for a 50L a change of 25 is a real problem but for big tanks this is nota problem, in special with some kinds of fishes like discus. Anyway u need to control the ph of the change and use calculators for it, adn with this u can control the PH. – Gawey May 2 '19 at 13:06

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