kH/pH crash, Mini cycle - The Planted Tank Forum
 
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post #1 of 10 (permalink) Old 01-22-2014, 03:32 AM Thread Starter
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kH/pH crash, Mini cycle

So my tank dropped to ~4.6pH this past week and 0kH. I've adjusted the pH back to 7.0 and kH 2. What could cause a kH crash like this? The pH of the tap water is even at around 8.6 and 3 dkH coming out of the tap (callibrated kits, pH probe).

As a result my tank went through a mini cycle.

How long do you guys think it will take to recover? Should I try to amend the water in the future from now on?

The tank is CO2, pfs substrate and planted with epiphytes.

Could it be that the CO2 caused a drop in kH and then subsequent crash?

Should I be using seachem acid/alkaline buffer ( I raised kH using alk buffer) or would a phosphate buffer be better?

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post #2 of 10 (permalink) Old 01-22-2014, 04:40 AM
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If co2 is causing your ph to drop below 6.0 it could create a situation where bacteria will become less effective. Also, if something died or is rotting in your tank the bacteria could have used up all the kh in the water resulting in a ph crash which makes things even worse.

50% water change and add a bit of baking soda to stabilize things. Never use phosphate buffers. Not only do they throw off co2 kh readings, but in a planted tank you dont want to slay your phosphate balance.
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post #3 of 10 (permalink) Old 01-22-2014, 04:51 AM Thread Starter
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Quote:
Originally Posted by Positron View Post
If co2 is causing your ph to drop below 6.0 it could create a situation where bacteria will become less effective. Also, if something died or is rotting in your tank the bacteria could have used up all the kh in the water resulting in a ph crash which makes things even worse.

50% water change and add a bit of baking soda to stabilize things. Never use phosphate buffers. Not only do they throw off co2 kh readings, but in a planted tank you dont want to slay your phosphate balance.

Don't measure co2. Just watch to plants really.

I'm just wondering if anyone has had this experience. Bacteria use kH?

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post #4 of 10 (permalink) Old 01-22-2014, 05:38 AM
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Yes. The nitrifying bacteria use carbonates as their source of carbon. Most organic processes such as decomposition, or composting (fallen leaves, fish poop, left over food...) will tend to have an acidic reaction.

I would test each batch of water as you are getting ready for a water change and add enough baking soda or potassium bicarbonate to make sure the KH is at least 3 German degrees of hardness. You might not have to add anything most of the time, but the tap water is not always the same.
Test the tank every few days through the week (or however long between water changes) to monitor what the KH is doing. You can add baking soda or potassium bicarbonate any time to boost the KH a degree or two.

Another possibility:
If the water company is adding something to keep the pH up (sounds like it with low KH but high pH out of the tap) perhaps something in the tank is removing that material from the water, then the low KH allows the pH to drop.
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post #5 of 10 (permalink) Old 01-22-2014, 05:52 AM Thread Starter
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Thanks for the information. I'll keep buffering the kH in future water changes.

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post #6 of 10 (permalink) Old 01-22-2014, 09:42 AM
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Diana, I have that situation with my tap water. Low kh ( about 1.5) and my ph out of the tap is like 8.5! What could the water company add to do this? Sorry for the thread hyjack

Tds out of the tap is 30ppm. My gh is only about 2. The only thing they could add that would do this is NaOH. It won't raise kh or gh, but would raise ph.

Last edited by Positron; 01-22-2014 at 10:02 AM. Reason: More info
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post #7 of 10 (permalink) Old 01-22-2014, 10:01 PM Thread Starter
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Can't really help you, positron. I think that municipalities add either NaOH or some type of phosphate based buffer to raise the pH. They do it to protect the pipes.


But I've been watching my ammonia...still stuck at 0.5ppm. Otherwise the pH and kH have been pretty much stabilized. Only time will tell what happens next.

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post #8 of 10 (permalink) Old 01-23-2014, 04:36 PM
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Theoretically CO2 cannot drop the pH that low. There is a limit on how low the pH can get from just dissolved CO2 in the water. Unfortunately I don't remember what that limit on pH is, but I believe it is above 5.0.

It isn't consistent for nitrifying bacteria to be limited in effectiveness by low pH, but still be able to consume carbonates until the pH drops to 4.6, which is well below the level at which the bacteria is considered to be ineffective at converting ammonia to NO2 to NO3. It would take a very big colony of bacteria to consume that much carbonate, but the low pH should have halted the expansion of the colony before the pH dropped that far.

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post #9 of 10 (permalink) Old 01-23-2014, 06:25 PM Thread Starter
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I'm still not so sure what happened. Ammonia less than 0.25 but not 0 right now.

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post #10 of 10 (permalink) Old 01-26-2014, 02:21 AM Thread Starter
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We've been at 0 and strong for the past 2 days. I had to dose additional KH on friday but since then it's been holding steady.

Thanks for the tips. I had no idea the bacteria needed KH.

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