Kh/ph Crashing


The fish tank I have crashed again. This time I didn't do anything abnormal. Just did regular water change once a week and topped off.
I noticed my fish looked off so I went to check the water. The water had low ph. 6.0 or lower and so I checked the other parameters. The tank went into a cycle again after 5 weeks. The kh is 0. I did a water change, within 6hours it was zero again. I have no idea what happened or why. It's in a hard crash and my poor fish is in the middle of it.
I'm not sure how to fix the issue. Buffering the ph/kh dip does nothing as it just reverts to 6.0/0 so fast I'm afraid I may lose my fish.
What would cause the kh to dip that fast? I've heard that kh can send the ph and water into a cycle.

What I mean by 0 is 1 drop and its yellow.


hi! so I might be able to help because we're on rainwater with 0kH/0gH. our tank crashed on new years and we lost 12/13 fish... 2 succumbed a month after because the ammonia spike to over 2 just took longer to kill the poor things.

what are you using to buffer?

we use 2 x fish safe mesh bags of oyster grit directly in the filter, & 1.5 cuttle fish bones in the tank. we also dose alkaline buffer, acid buffer & Equilibrium to the volume of new water every weekly water change. we're stable now and have been for 6 months.

edit: the reason for your crash is because kH buffers your water's pH meaning it keeps your pH stable. 0kH means you've got no buffer so every little thing causes your pH to swing. if you oxygenate your waster or have driftwood, your pH can plummet.

if your pH plummets below 6, your BB can have trouble growing. too far below 6 and your BB die off. that's what causes the tank to cycle.


You received some good advice from Jenoli42 above.

As stated, when you have a low KH, your pH is extremely susceptible to change (and it always seems to be a change down).

At a pH below 7.0 the ammonia in your tank starts turning into ammonium, and by the time your pH get down to 6.0, all ammonia has turned into ammonium. The good thing about ammonium is that it is far less toxic to fish than ammonia is (some claim it is non-toxic). The bad thing about ammonium is that it is a horrible food source for your ammonia converting bacteria.

So if you have a cycled tank, and the pH drops down close to 6.0 (or below) all the ammonia is now ammonium. But since the ammonium is a poor food source for the ammonia converting bacteria, it starts to starve. Initially the bacteria will become dormant, but if the pH is low enough, long enough the bacteria will starve off and die.

But the problem with the bacteria becoming dormant is that it is not longer converting anything into nitrites. So now your nitrite converting bacteria has no food source. Apparently this bacteria does not become dormant, before dying off, it just starves off.

So lets say your ammonia converting bacteria is dormant due to your pH not being too low for so long it starved off. Now you perform a partial water change with water that has a higher pH. Now the pH of the water in your tank rises. (Say your tank water had dropped to 6.0 and you did a 50% water change with tap water that had a pH of 7.0...the pH of the tank would now be 6.5). Suddenly as the pH moves closer to 7.0 that not so toxic ammonium starts turning into highly toxic ammonia. But that dormant ammonia converting bacteria now has food, wakes up and starts consuming that ammonia and releasing toxic nitrites. But sadly, because nitrites were not being produced while the bacteria was dormant, the nitrite converting bacteria died off and you now have a huge nitrite spike on your hands. This happened to me a few years ago and wiped out my entire tank.

So what you'll want to do is get your KH higher. The method I chose was to put some crushed coral into my filter. It is a trial end error process. So what you do is add say a 1/2 cup of crushed coral, wait 24 hours, test your KH. If good, you're good, if not add maybe 1/4 cup of crushed coral, wait 24 hours test...etc, until you get your KH where you want it.

You can put this crushed coral into a media bag and stuff the bag in your filter if it will fit. If not you can put the bag right into the tank. But if you do not like that look you can just sprinkle it in with your gravel.

Hopefully I explained this in an understandable manner, but if you have any questions, feel free to ask.

Best of luck!

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