Water change practice

planter

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Thanks for your explanation Clive

I must be the only thicko on the forum who has had this much trouble getting his head round this :oops:
You will forgive me but I never did do chemistry.

You mean to tell me that ive been lugging around bloody great barrels of RO water for nothing?? lol
I tell you what im going to do - next tank (coming soon) im gonna fill it with tap water! and use it as a learning exercise.

Perhaps I should re install my drop checkers and stop guessing?
Ill stick to the aquascaping while you guys keep me up to speed on the chemistry.

And thanks Sam you may have just saved me a fortune with the chiropracter :D .
 

Themuleous

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Sorry to have started something! My intention wasn't to point out a fault Planter :) just that if you are using RO purely to reduce the amount of CO2 you are adding then you're making work for yourself :)

It is bloomin' confusing I must admit but I think this is what is most important

Neither stability of pH, nor KH can affect the ability of the water to dissolve CO2. Therefore, you should not have to change the injection rate of the CO2 just because you have a higher KH.

I guess the other thing to remember is that the pH scale is logarithmic i.e. pH 6 is 10x more acidic than pH 7 and similarly, pH 5 is 100x more acidic than pH 7. Im no chemist but its probably this fact that accounts for the apparent 'fixed' relationship between CO2, pH and KH. (But I could be wrong :LOL:)

Sam

EDIT - alls well that ends well ;)
 

planter

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Themuleous said:
Sorry to have started something! My intention wasn't to point out a fault Planter :) just that if you are using RO purely to reduce the amount of CO2 you are adding then you're making work for yourself :)

It is bloomin' confusing I must admit but I think this is what is most important

Neither stability of pH, nor KH can affect the ability of the water to dissolve CO2. Therefore, you should not have to change the injection rate of the CO2 just because you have a higher KH.

I guess the other thing to remember is that the pH scale is logarithmic i.e. pH 6 is 10x more acidic than pH 7 and similarly, pH 5 is 100x more acidic than pH 7. Im no chemist but its probably this fact that accounts for the apparent 'fixed' relationship between CO2, pH and KH. (But I could be wrong :LOL:)

Sam

EDIT - alls well that ends well ;)

Im glad you did start something Sam, No pont using a forum that doesnt help!
 

mick b

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Excellent explanation, thanks.

Coming from the Discus side of things, CO2 tends to be considered as a Ph reducing tool as it promotes the take-up of KH, thus reducing Ph, hence the parallel with using RO to reduce TDS (including kH) again to drop Ph

I learned someting today, Cheers, Mick B :D
 

Ed Seeley

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I'll wade in here as I use RO too. I use it as I keep soft water fish and find I get better results with plants and fish using it. I don't waste any water as the 'waste' feed goes straight into my koi pond!

One thing koi keepers have found is that koi grow much faster in softer water than harder and also changes the way their colours develop so there may be more to using soft water for naturally soft water fish like koi than simply survivability and breeding.
 

ulster exile

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For those people who use a pump or other semi-automated system, what do you do about cleaning the substrate? I tend to leave it alone, as most of the tank's substrate is covered and I presume that the build up of mulm will help the plants but having religiously gravel vac'd for about 2 years, leaving the substrate just feels 'wrong' :?
 

JamesC

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Planter was correct all along regarding disolving CO2 in water with varying KH's. The higher the water's KH the more difficult it becomes to dissolve CO2 in the water and hence the more CO2 is needed to be injected to compensate for greater CO2 loss. It's not a great difference but it is detectable.

James
 

plantbrain

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However, whether the CO2 gas is dissolved or not may not matter at all, all that matters is that enough CO2 gets to the plants.

If you add say 3 bubbles per second in a a tank with a KH of 10 and another with a KH 1, you still add exactly the same amounts of CO2 gas, pH is not even considered here, go directly to CO2.

A ph/Kh combo of 6.0 and 1 = 30ppm
A pH/KH combo 7.0 and 10 = 30ppm

It takes the same amount of CO2 gas to get 30ppm from ambient conditions, yes, the CO2 can have differing solubility at different KH's, however, this is not going to make any real practical differences to plants or the CO2 system.

It does make some differences in natural systems where CO2 is not enriched like we do it often in CO2 planted tanks, subtle changes can make differences there, but not in our aquariums.

This is where practical applied knowledge and consideration comes into play versus trying to compare things to natural systems and relationships. While true is some regard, it has virtually no effect near as I can tell using a a wide range of KH's and CO2 meters over the years. I've had less than 1 Kh to upwards of 12 degrees for several years at a time, a few plants are weak at higher KH's, but it had nothing to do with CO2.

Temperature decrease also increases the ability for water to hold more dissolved gas(all, not just CO2) as well.
Higher you go, the more steam and boiling water vapor you get.

I think this is a big issue for folks that tend to use RO water/and keep Discus, they tend to use higher temps, then have large fish, all issues when it comes not only to CO2, but O2 as well.

To increase O2, often they increase the wet/dry filters, add more surface turn over, but this has nothing to do with KH does it?

Nope, not one bit.

I know much more about saltwater systems and CO2 unfortunately, but the balance of these carbonate species (which ultimately affects the solubility of carbon dioxide), is dependent on factors such as pH. In seawater this is regulated by the charge balance of a number of positive (e.g. Na+, K+, Mg2+, Ca2+) and negative (e.g. CO32- itself, Cl-, SO42-, Br-) ions. Normally, the balance of these species leaves a net positive charge. With respect to the carbonate system, this excess positive charge shifts the balance of carbonate species towards negative ions to compensate. The result of which is a reduced concentration of the free carbon dioxide and carbonic acid species, which in turn leads to an oceanic uptake of carbon dioxide from the atmosphere to restore balance. Thus, the greater the positive charge imbalance, the greater the solubility of carbon dioxide. In carbonate chemistry terms, this imbalance is referred to as alkalinity.
So harder water will have higher solubility of CO2.
CO2 dissolved in the ocean immediately reacts with water to form carbonic acid. Most of the carbonic acid gives up a
proton (H+)…dissociates…to form bicarbonate. Some of the bicarbonate dissociates to form carbonate. Thus harder water holds more or put another way, CO2 is more soluble in harder water.

As far as use of CO2 required for a tank, this is another area.
Refer to Fick's 1st law of diffusion.

This is really the biggest issue for us, not so much the KH, that's more for certain species of plants and fish.
Diffusion is the biggest issue and the ability for the CO2 to get to the plants is the primary driver here.

More current = more off gassing, however it also means less boundary layers and better CO2 delivery/mixing and harder for many species fo algae to attach, less detritus on leaves providing fuel for algae and other periphyton growth.
More current also means more stable O2 levels and often higher levels.

So this is a trade off, one long ignored and neglected at the expense of the fish and plants.
We can add a tad more CO2 and get the benefits of more stable O2, better CO2 delivery, better mixing and less algae/dirt in our tanks.

This seems to be a good trade off to me.
If you can, a lower KH is nicer for more plant species, most every hard water plant can do very well in soft CO2 enriched aquariums(I've not found one to date that cannot do well). But low KH is not going to save folks from algae or CO2 woes.

Just keep that in mind there.

CO2 and alkalinity are the two least understood topics in this hobby.

Even back in the mid 1990's folks where telling others to watch how they measure CO2 and the KH issues:
http://www.thekrib.com/Plants/CO2/hardn ... frank.html

Read bottom of the page. I had pretty high PO4's in my tap back then and ignored the pH/KH and went with fish and plant responses for 30ppm(later determined using RO reconstituted).

Hope this helps, likely confuses, but the bottom line is worry more about the Fick's 1st law than solubility.
As far as fish and perhaps a few plant species, focus on lower KH's 3-5 seem okay for most so called low KH plants.

Folks that claim super low pHs and low KH's in the 5.5-6.0 ranges or KH's at 1 or so are required for any plant full of beans. Look for advice elsewhere.

Regards,
Tom Barr
 

JamesC

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All very interesting but it doesn't get away from the fact that in an open system it requires more injected CO2 to hold a constant CO2 concentration in a high KH water than in a low KH water.

Does it really matter that much? No, not really. Just increase bubble rate a bit.

James
 

aaronnorth

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If you add say 3 bubbles per second in a a tank with a KH of 10 and another with a KH 1, you still add exactly the same amounts of CO2 gas, pH is not even considered here, go directly to CO2.

yes you add the same amounts of CO2 gas, but less of it will dissolve in the tank with a KH of 10 so more will need to be injected to reach 30ppm wont it?

thanks, aaron

(its probobaly just me getting confused again :rolleyes: )
 

ceg4048

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What Tom is saying is that even at very high KH the solubility of CO2 is still very high. You won't see much of a difference in the injection rates and it most certainly doesn't warrant bucketloads of RO water, that's for sure.

The common misconception is that the tank water should be driven to the same pH regardless of it's KH. This is not true as I tried to illustrate with Chucks table. For all intents and purposes you should consider the solubility the same. As James points out, you may need a minor adjustment to bubble rate but the difference will not be significant.

CO2 in the tank water comes out of solution into the air bubble in the dropchecker, then diffuses into the dropcheckers sample water. The change in pH of this dropchecker water is different than that of the tank because it is has a KH of 4. When the dropchecker turns green (pH 6.6) this tells you that the concentration in the dropcheker's water is 30 ppm. This also implies that the tank water has 30 ppm although the tank water's pH can be totally different based on it's KH and it's acid content.

Another thing to remember is that only a tiny fraction of the CO2 that is injected in the water actually reacts to form carbonic acid. We see this reflected as a pH drop but the vast majority of the gas stays as CO2.

Cheers,
 

ceg4048

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Yeah, that's the bottom line. There are folks out there with KH even above 18 and they don't report any difficulty injecting CO2. There are a lot more things that affect CO2 uptake to the plants than high KH, such as poor flow or inefficient delivery methods, or even insufficient injection rates.

Now, as Tom indicated, there are plants that don't like high KH, but these are only a handful. So if the plant doesn't grow well in high KH water it's not because CO2 doesn't dissolve well, it's because that plant doesn't like high KH.

Cheers,
 

Egmel

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ceg4048 said:
Yeah, that's the bottom line. There are folks out there with KH even above 18 and they don't report any difficulty injecting CO2.
Will second this, I have a KH of 15-17 and on my 70l I can get ~30ppm with DIY! (when it's not leaking :rolleyes: )
 
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