Kh, ph and fish safety - The Planted Tank Forum
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post #1 of 77 (permalink) Old 08-07-2019, 05:23 AM Thread Starter
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I'm going to leave out details in order to keep this short - I'm stretched for time. Can fill things in later.

I just did a water change with RO water and didn't add any buffering. Figured this would work great with whatever is already there (kh around 2.5). Therefore expecting my tank kh to get down to 1.5ish.

Test kits (api and salifert) are both saying kh is below 1. Not good, right? According to Salifert, my kh is about .6. I've double-checked that result once.

I did the water change and maintenance about 2 or 3 hours ago, noticed my problem and immediately put some KHCO3 into some separate water. I know this stuff takes some time to mix.

I want to go to bed... It's late... But I don't want to hurt any fish 😞

Can someone give input? Last thing I want is to kill off my entire tank because of pH swings due to no kH.
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post #2 of 77 (permalink) Old 08-07-2019, 05:54 AM
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Kh, and therefore ph go up very fast after adding it. If you use a ph pen you can watch your ph increase after you dump it in. 15 minutes to mix, max.

Also, why would 0.6 kh be harmful to your fish? Are you injecting a ton of co2 in? If so, hold off until you get your kh and ph back up to whatever normal is for you.

I killed my shrimp by adding way too much kh once. They will be freaking out and swimming all over the place. Fish didn't even notice.
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post #3 of 77 (permalink) Old 08-07-2019, 06:03 AM Thread Starter
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Originally Posted by Ddrizzle View Post
Kh, and therefore ph go up very fast after adding it. If you use a ph pen you can watch your ph increase after you dump it in. 15 minutes to mix, max.

Also, why would 0.6 kh be harmful to your fish? Are you injecting a ton of co2 in? If so, hold off until you get your kh and ph back up to whatever normal is for you.

I killed my shrimp by adding way too much kh once. They will be freaking out and swimming all over the place. Fish didn't even notice.
I'm under the impression a .6 kh would allow for drastic pH swings and potentially kill fish.
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post #4 of 77 (permalink) Old 08-07-2019, 06:22 AM
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A IV drip setup or a leach box are a very useful things to have around.

To make a leach box just get standard specimen cup that hangs on rim of tank, drill a hole about 1/16” in both left and right end and set in tank where current gently flows by it. Mix your stuff with enough water to dissolve it and poor it in. Tiny bit of current flowing through it will slowly leach solution into tank over a few hours.
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Last edited by DaveKS; 08-07-2019 at 06:59 AM. Reason: typo
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post #5 of 77 (permalink) Old 08-07-2019, 12:06 PM
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I'm under the impression a .6 kh would allow for drastic pH swings and potentially kill fish.
My KH has been under 1.0 for some time now.

Fish are great, no pH swings.

And plants love the soft water.
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post #6 of 77 (permalink) Old 08-07-2019, 01:35 PM
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I was running 0.5 degrees of kH and mixing it < 24 hours before a water change. Plants and fish were perfectly fine. pH never crashed and stayed steady.

Albeit I had a small amount of seiryu stone in the tank..... which kept pH about 7.2 degassed and kH a littler higher than 0.5 in the tank despite my incoming RO water being 0.5.


gH was boosted to 7 or so... API kit took a solid 8 drops, I would assume the seiryu stone is the culprit again for a slight raise in hardness.

Nonetheless kH < 1 is safe. Just be sure to be consistent for the sake of the fish/inverts/plants.
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post #7 of 77 (permalink) Old 08-07-2019, 03:19 PM
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My situation is identical to @Greggz: dKH less than 1, CO2 ~35 and pH stable in the 5.5.-6.3 range (never fully degasses). Plants are thriving and I have a very colorful and active range of fish from tetras up through live bearers, but no shrimp. Plus, the benefit of organics never being in the ammonia form.

@Quagulator hit upon the key: consistency. Set a pattern and hold it. TDS is much more important than pH.
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post #8 of 77 (permalink) Old 08-07-2019, 05:38 PM
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Im confused--- some questions:

Tell me if Im getting what y'all are saying correctly: It doesn't matter what the actual number of KH is when it comes to fish in general ( the OP was concerned about his fish), it is just important that it remain consistent ( even at .5-1 ppm) In other words, in the aquarium, whether you are keeping cichlids from the Rift Lakes of Africa or discus from acidic black-waters of Rio Negro in Brazil, the actual number of KH is irrelevant ( as long as it remains stable).

Is a KH of .5-1 ppm a number you would recommend to all levels of aquarists? Beginners as well? I was under the impression that the lower the KH number was, the harder it was to keep that number consistent due to lower buffering capacity. You all make it sound so easy- like a KH number of .5-1ppm poses no risks to any level of aquarist.

If plants were not in the equation, an aquarist had a fish only tank, would a .5-1 ppm KH be a number you would promote as universally fine with all fish ( regardless of their geographic origins) as long as kept stable?
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Last edited by Discusluv; 08-07-2019 at 06:25 PM. Reason: more info
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post #9 of 77 (permalink) Old 08-07-2019, 06:43 PM
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Originally Posted by Discusluv View Post
If plants were not in the equation, an aquarist had a fish only tank, would a .5-1 ppm KH be a number you would promote as universally fine with all fish ( regardless of their geographic origins) as long as kept stable?
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Nope, whatever cards life has dealt you (speaking geography/tap water source here) within reason is what is likely fine for most common aquarium fish.

Discus being an exception of course liking soft acidic water, and Africans being at the opposite side of the spectrum liking hard, alkaline water. I encourage a beginner to do lots of research and ask as many questions as they feel they need to. Watching a beginner's success is super rewarding if they so choose to share their journey and ask questions / follow sound advice.

Generally speaking, 90%-95% of commonly available fish are fine in a large range of tapwater sources. Mine being very hard, never had an issue with fish before. I'm confident my fish would be perfectly safe in 0.5 degrees of kH or 15 degrees of kH (because that's exactly what they have experienced within the last 6 months with me changing from RO water back to tapwater). Absolutely 0 issues.

But, if source water is <1 degree, I would say er' on the side of boosting it to ~1 and not 0.5 degrees. This is of course speaking for most common aquaria fish species, if African Chichlids / mollies I would verge on going a little harder, or using some sort of buffer (crushed coral, Texas holy rock, limestone etc etc).

But, since this is a planted tank forum, I'll speak for myself here, I will always aim my fish keeping advice keeping plants in the cross-hairs. It's generally accepted most, if not all, plants prefer soft water, and because RO is becoming so much more popular, targeting the lowest "safe" kH range is likely the best advice for the wallet, for the ease of maintenance, and for the well being / best chance of a successful planted tank.
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post #10 of 77 (permalink) Old 08-07-2019, 06:47 PM
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Can't say I'm as experienced as some of those commenting above BUT I do have experience with low kH leading to pH swings, leading to mass fish deaths.

If I were you, I'd slowly raise kH back to 2.5dkH or whatever you're aiming for. When changing water in future, ensure all water added matches the water in the tank (excluding top offs). 2.5dkH should be enough to keep your pH stable without any hassle.

Not sure if you guys have it over in the USA, but JBL Aquadur is godsend. No need to mix hours in advance, just mix into the water before adding and give it a stir. Brought my tapwater up from <1dkH to 2dkH, gH from 3dgH to 5dgH, and keeps my pH at a level 7.2 instead of swinging from 7.2 to 6.

I don't inject CO2, I don't use a TDS pen, and I'm pretty basic when it comes to this stuff, but in my opinion it would be better to stick to 2.5dkH in order to minimise risk to livestock. pH shock is a horrible way for a fish to die, and although my plants aren't on the same level as some of those above, I highly doubt it's down to kH.
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post #11 of 77 (permalink) Old 08-07-2019, 06:51 PM
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Originally Posted by Quagulator View Post
Nope, whatever cards life has dealt you (speaking geography/tap water source here) within reason is what is likely fine for most common aquarium fish.

Discus being an exception of course liking soft acidic water, and Africans being at the opposite side of the spectrum liking hard, alkaline water. I encourage a beginner to do lots of research and ask as many questions as they feel they need to. Watching a beginner's success is super rewarding if they so choose to share their journey and ask questions / follow sound advice.

Generally speaking, 90%-95% of commonly available fish are fine in a large range of tapwater sources. Mine being very hard, never had an issue with fish before. I'm confident my fish would be perfectly safe in 0.5 degrees of kH or 15 degrees of kH (because that's exactly what they have experienced within the last 6 months with me changing from RO water back to tapwater). Absolutely 0 issues.

But, if source water is <1 degree, I would say er' on the side of boosting it to ~1 and not 0.5 degrees. This is of course speaking for most common aquaria fish species, if African Chichlids / mollies I would verge on going a little harder, or using some sort of buffer (crushed coral, Texas holy rock, limestone etc etc).

But, since this is a planted tank forum, I'll speak for myself here, I will always aim my fish keeping advice keeping plants in the cross-hairs. It's generally accepted most, if not all, plants prefer soft water, and because RO is becoming so much more popular, targeting the lowest "safe" kH range is likely the best advice for the wallet, for the ease of maintenance, and for the well being / best chance of a successful planted tank.
The OP did not ask about their plants, they asked about/were concerned about their fish- regardless if this is a forum for planted tanks.



Thanks for your answer, although it did not directly address my questions.


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post #12 of 77 (permalink) Old 08-07-2019, 06:51 PM
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Can't say I'm as experienced as some of those commenting above BUT I do have experience with low kH leading to pH swings, leading to mass fish deaths.

I'm very curios to here your experience / story of a pH crash. I've never seen it, or followed a thread regarding it happening - I would like to see some more light shed on it TBH.
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post #13 of 77 (permalink) Old 08-07-2019, 07:02 PM
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IMO, if plants weren't in the equation, I would select a KH that would deliver the nominally optimal pH for the fish I had and that is only for the sake of consistency. I would likely target 6.8 as a pH level (you have to pick some level), just to be sure of ammonium vs. ammonia. It's tough to shake the idea of pH being so important, having so much early history that enforces it, but I believe that tropical fish are imperceptibly tolerant in a wide pH range (5-8+) and that TDS dwarfs PH in importance. Inconsistency in TDS is far more stressful to fish than inconsistency in KH / pH.

I don't keep Discus, but am I right that their natural habitat is soft water both in terms of KH and GH?

So, if someone held a gun to my head and asked if a dKH (I think you meant degrees and not ppm) of .5-1.0 would be "universally fine" for all tropical fish, I'd say: yes, but I'd also say yes at 2 or 3 or x, so long as it provided stability and had very little impact upon TDS. I would have to say: "Then pull the trigger", if asked what the best KH level is for all tropical fish.
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post #14 of 77 (permalink) Old 08-07-2019, 07:04 PM
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Originally Posted by Discusluv View Post
The OP did not ask about their plants, they asked about/were concerned about their fish- regardless if this is a forum for planted tanks.



Thanks for your answer, although it did not directly address my questions.
I'm not exactly seeing any contribution on your behalf to OP's question?

OP is using RO water, it's been proven by many users on here targeting 0.5 - 1.0 degrees (not ppm) of kH is perfectly safe and quite capable of avoiding pH swings.

@jcoulter , we should have asked which species of fish you are keeping before recommending a low kH.

I'll toss $20 on the line and say it's not Rift Lakes of Africa or discus from acidic black-waters of Rio Negro in Brazil
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post #15 of 77 (permalink) Old 08-07-2019, 07:21 PM
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Answer is pretty basic- rapid drops in pH can be deadly to fish. Its a foregone conclusion. In addition, the lower your KH, the more risk involved that, through denitrification processes in the aquarium, a crash will happen. This occurrence most likely presenting a problem to unsuspecting ( beginning aquarists) those who have little experience in maintaining, monitoring, and understanding these processes in the aquarium.
The 1KH that is promoted as fit for 95% of fish as a general theory of approach when your focus is the planted tank doesn't hold up as true when you look at it through the lens of a fish- only system. So, there appears to be some "focus" discrepancies here.
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