Higher KH to save CO2? - The Planted Tank Forum
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post #1 of 10 (permalink) Old 08-17-2017, 06:28 PM Thread Starter
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Higher KH to save CO2?

My tank's KH drifted from 4 to 8 over the past couple months.

Turns out that I need half as much CO2 to maintain the same 30 PPM. If I decide to run 15-20ppm, my tank could last 4x as long.

I have been adding Alkaline buffer to bring KH to 4-5. Tap is KH 2. Something with my Ei dosing or perhaps the MGOC substrate raised KH higher than I thought.

Is there any reason not to keep a KH of 8? I dont have any demanding plants or fish afaik. Are there some plants that do worse in KH 8 as opposed to 4? I dont want to go lower than 4 to avoid ph swings.
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post #2 of 10 (permalink) Old 08-17-2017, 07:19 PM
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Originally Posted by ChrisX View Post
My tank's KH drifted from 4 to 8 over the past couple months.

Turns out that I need half as much CO2 to maintain the same 30 PPM. If I decide to run 15-20ppm, my tank could last 4x as long.

I have been adding Alkaline buffer to bring KH to 4-5. Tap is KH 2. Something with my Ei dosing or perhaps the MGOC substrate raised KH higher than I thought.

Is there any reason not to keep a KH of 8? I dont have any demanding plants or fish afaik. Are there some plants that do worse in KH 8 as opposed to 4? I dont want to go lower than 4 to avoid ph swings.
Where are you seeing that it takes half as much? the CO2 kh ph chart?
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post #3 of 10 (permalink) Old 08-17-2017, 07:31 PM
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If it is the chart you are referring to I believe you are mistaken. Think of pH as the output. I know the chart looks like if you change any of the variables the CO2 will change, but this isn't how it works because both other variables will change at the same time. If you simply increase kH, your pH will go up as well. And if they both increase you won't actually be getting more CO2.

If you have a higher kH for instance, it will take more CO2 to get the same change in pH than if your kH was 4 for example. After all kH is buffering through carbonate, so at a higher kH it is easy to see that it takes more carbonic acid to reduce the overall pH.

I think Tom Barr said it best "The only way to add more CO2 is to add more CO2."

The chart is just a way for you to determine how much CO2 is in the water given your kH and current pH.
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post #4 of 10 (permalink) Old 08-17-2017, 07:53 PM Thread Starter
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If it is the chart you are referring to I believe you are mistaken. Think of pH as the output. I know the chart looks like if you change any of the variables the CO2 will change, but this isn't how it works because both other variables will change at the same time. If you simply increase kH, your pH will go up as well. And if they both increase you won't actually be getting more CO2.

If you have a higher kH for instance, it will take more CO2 to get the same change in pH than if your kH was 4 for example. After all kH is buffering through carbonate, so at a higher kH it is easy to see that it takes more carbonic acid to reduce the overall pH.

I think Tom Barr said it best "The only way to add more CO2 is to add more CO2."

The chart is just a way for you to determine how much CO2 is in the water given your kH and current pH.
50g tank has a Griggs reactor and 24oz PB tank.

I have halved the CO2 output from 3bps to 1.5bps (large bubbles), and everything is still bubbling and pearling. Fish respiration is normal. Today, pH is 7 as opposed to 6.6 before. At night it goes to 8.1. IME, raising KH 3 points using AB only raises PH .1 point.

My observation is that the extra Alkaline Buffer I have been adding, is much less expensive than the cost of extra CO2 and saves me trips to Dick's for refills.

If I keep the tank set up as is, according to the chart, I am at 30ppm and using just half the CO2 I was using before. Which means I will get 2 months from the PB tank. If I decide to run at say 15-20ppm, I will possibly get 3-4 months!

My point is that it costs much less to keep Kh high than it does to keep KH low and use 2x the CO2. Also, the PH stays closer to neutral and is possibly less stressful for fish.
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post #5 of 10 (permalink) Old 08-17-2017, 08:53 PM
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Originally Posted by ChrisX View Post
50g tank has a Griggs reactor and 24oz PB tank.

I have halved the CO2 output from 3bps to 1.5bps (large bubbles), and everything is still bubbling and pearling. Fish respiration is normal. Today, pH is 7 as opposed to 6.6 before. At night it goes to 8.1. IME, raising KH 3 points using AB only raises PH .1 point.

My observation is that the extra Alkaline Buffer I have been adding, is much less expensive than the cost of extra CO2 and saves me trips to Dick's for refills.

If I keep the tank set up as is, according to the chart, I am at 30ppm and using just half the CO2 I was using before. Which means I will get 2 months from the PB tank. If I decide to run at say 15-20ppm, I will possibly get 3-4 months!

My point is that it costs much less to keep Kh high than it does to keep KH low and use 2x the CO2. Also, the PH stays closer to neutral and is possibly less stressful for fish.
Alkaline buffer is carbonate based and it makes sense that your degassed pH is around 8.1 Increasing kH with a carbonate based buffer will only increase your pH up to about 8.2, after that your kH will continue to go up with little to no effect on your pH.

As to stress on your fish: pH changes as a result of CO2 do not affect your fish. This is why people are able to swing their pH a full point daily with no effects on fish. You can look this up more, but the reason is that adding CO2 isn't changing the mineral content of the water. So it won't effect osmoregulation etc. The "real" pH relevant to your fish though is 8.1. Even if during the day it says 7.

If your plants are pearling, your plants are pearling. Do what works for you. I'm pretty sure plants consuming kH as a carbon source isn't as efficient as CO2 as a carbon source but in the grand scheme of things I don't think that matters much I guess.
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post #6 of 10 (permalink) Old 08-17-2017, 09:07 PM Thread Starter
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Maybe one of the benefits of having KH 2, pH 8 tap water.
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post #7 of 10 (permalink) Old 08-17-2017, 09:11 PM
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As an aside, if you doubled your kH to 8 and halved the CO2 you are putting in, I would bet almost anything that your pH due to carbonate hardness is actually closer to 7.2 than 7.0

If you halved the CO2 going in I can guarantee your CO2 concentration in the water is half.
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post #8 of 10 (permalink) Old 08-17-2017, 09:42 PM Thread Starter
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As an aside, if you doubled your kH to 8 and halved the CO2 you are putting in, I would bet almost anything that your pH due to carbonate hardness is actually closer to 7.2 than 7.0

If you halved the CO2 going in I can guarantee your CO2 concentration in the water is half.
Well, that's ok. It was close to 60ppm according to the table and fish respiration was much faster. IDK if I'm missing your point.

My point was that because the KH had somehow crept up, the CO2 was much higher than it needed to be... I bet you feel like we're going in circles.. Thx for your help.
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post #9 of 10 (permalink) Old 08-17-2017, 09:57 PM
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Well, that's ok. It was close to 60ppm according to the table and fish respiration was much faster. IDK if I'm missing your point.

My point was that because the KH had somehow crept up, the CO2 was much higher than it needed to be... I bet you feel like we're going in circles.. Thx for your help.
Oh no its fine. I'm glad its working for you. Happy plants are happy plants. I guess the overall point I'm trying to make is this: I don't want people to think that by simply increasing kH they can use less CO2 and maintain the same concentration of CO2 in the water. This is a common mistake new people make using the CO2 kh ph chart and I was just clarifying.

I realize now that you aren't saying that, you are just saying that your kH was high and your pH was staying the same so you had more CO2 than you needed/thought you did.
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post #10 of 10 (permalink) Old 08-18-2017, 02:06 AM
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Why are you worried about kH?
Many of us stopped adding it years ago.
I would suggest stop measuring it and instead, aim for a pH change of 1 with your CO2.
You may not need that much CO2 if you do not have high light.
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