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Can you have low ph with high alkalinity ?

anewbie

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I've gotten myself confused over alkalinity and ph so I guess I have two questions. First do fishes that require low ph (4.5-5.5) also require a certain alkalinity and if so what alkalinity. Second in a low ph aquarium (using ro water); do i want to boost alkalinity with limestone or similar for stability ?
 

Ben Rhau

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Most ways of boosting alkalinity will raise pH. The only way I know of to have both high alkalinity and low pH is through phosphate buffers. However, this will produce a sky-high TDS. Not recommended.
 

Ben Rhau

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Short answer is no. I'm not sure what an alkalinity crash is or what the effect would be. My tanks have a KH of zero.

It goes back to why those fish come from areas of low pH. They don't come from water that's highly buffered at low pH. The pH is low because it's soft.

Are you afraid the pH will crash? It doesn't happen. It's REALLY hard to keep the pH low, because dissolved CO2 creates carbonate. Without adding acid, the pH wants to be at 5.5 or 6.
 

anewbie

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Someone warned me that i needed to watch alkalinity and the fluctuation in alkalinity was dangerous to blackwater fishes. They recommended I put some limestone in the tank but that seemed questionable since it would raise ph hence i posted my question.
 

Ben Rhau

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By alkalinity, do you mean KH? The KH of blackwater is zero, so it won't fluctuate.

Conventional wisdom says that pH will fluctuate a lot if there's no buffering. In practice, I've not seen this to be the case, for the reason I stated above. Having spoken to many blackwater keepers, The only ways to get the pH below 5.5 are:
  1. Peat.
  2. Lots of organics continually added over a very long period of time.
  3. Strong acid.
The pH does not crash. Most people have the opposite problem, where the pH is higher than they want. I tried to keep the pH at 4.5 without peat, and I couldn't do it without adding a lot of strong acid at every water change. I don't like doing that, because I think the differential of new water vs current tank water is too high. So I keep my tanks at around 5.5. It doesn't fluctuate much at all. It drifts up very slightly over time, but I add a (very small) amount of acid to the incoming water to keep the carbonic acid-carbonate equilibrium low.
 

Ben Rhau

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Now, if you add CO2, that will cause the pH to fluctuate. I don't know many people who add CO2 to blackwater tanks. Most blackwater habitats are pretty devoid of plants, but I can still get the really hardy low tech ones to grow.

I have a thread about my setups <here>.
Since I'm not keeping cichlids, I redacted <the thread for this forum> to only talk about the water chemistry.
 

anewbie

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He was claiming that even if ph was stable alkalinity will fluctate and it was the alkalinity changes that was harmful to the fishes.
 

Ben Rhau

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Alkalinity is carbonate. If you don't add any supplemental carbonate, there will be some carbonate from CO2 that exists in equilibrium with carbonic acid. But it's so low that the KH (carbonate hardness) still reads zero. So he's saying the fluctuation from one tiny number to another tiny number is harmful to fish? Does he say how? I don't see the logic here.
 

MacZ

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I can also not think of a situation where KH would fluctuate in a way it's dangerous without it taking the pH with it for the ride.
And I have also no idea what a KH crash is supposed to be. A quick search showed some hits in context of saltwater tanks but it seems only to be relevant for reef tanks.

So I have to presume that person who told you has no idea what they're talking about.
 

Ben Rhau

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You would either need to start with a high KH and then have some mechanism to neutralize it quickly, or add carbonate ions and water change it out. It won’t spontaneously fluctuate in a zero KH tank.
 

dw1305

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5 Year Member
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Wiltshire UK
Hi all,
Someone warned me that i needed to watch alkalinity and the fluctuation in alkalinity was dangerous to blackwater fishes. They recommended I put some limestone in the tank but that seemed questionable since it would raise ph hence i posted my question.
Just ignore them, they are totally clueless.

In fact when you aren't on this forum just ignore what anybody tells you about pH, hardness, pH crashes and buffering, because it will all be wrong, very wrong or totally wrong.

Cheers Darrel
 

MacZ

Well-Known Member
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Location
Germany
In fact when you aren't on this forum just ignore what anybody tells you about pH, hardness, pH crashes and buffering, because it will all be wrong, very wrong or totally wrong.
I wasn't going to say it that way, but probably I should have. So I'll at least expand on this:

Outside circles that do softwater (dwarf cichlids, wild bettas, wild gourami, wild discus, biotopes...) basically everybody believes pH will fluctuate in soft water, there is no buffer besides KH and a pH crash is a real danger.

The pH doesn't fluctuate constantly and in dangerous amplitude in relation to time in soft water.
There are alternatives to the KH buffering system.
There is no constant danger of a pH crash in soft water.
The beneficial microorganisms that keep the cycle going do not require a pH above 7.

All misconceptions fired by marketing, old school hobbyists that never bothered to evolve (let alone learn anything about water chemistry) and over and over info from regurgitated beginner's guides from the 50s and 60s.
 

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