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Hi,
I am currently quite confused about the relationship between CO2, pH, and KH. I have an alkalinity test kit where you perform a titration. The beginning colour is blue and the end point is light green. I used one drop and the colour went from blue to yellow. I drop = 0.5meq/l.
1meq/l = 50ppm CaCo3 = 2.8 degrees german hardness.
my pH is 6.0 and i use rain water.
I started adding some DIY CO2 to see if it boosted plant growth about 3 days ago.
I am not sure of the consequences of adding Co2 with a low pH and (i guess) low KH in regards to the health of my fish. What is considered high KH? Will my pH continue to fall? Any info would help as i'm kinda confused at the moment. Thanks!

If you have 50ppm of KH, then you are fine, add enough CO2 to reduce the pH to 6.4, keep it there during the light peroid.

It's doesn't sound like your KH is 50ppm though.
You can double the volume of the smaple water, this will 1/2 the reading and allow you a better accuracy at lower range.

So if the sample size is 5mls, use 10mls, then divide by two to get the reading.

We add CO2 not to directly control pH, we add it to fertilize the plants and allow them to grow at a faster rate. pH is just the measurement with KH to determine CO2.

A problem may occur, I'm not sure it is a problem personally when the KH is 5ppm or less and you no logner have any buffer. In high light tanks and if you want to detrmine CO2, this becomes tough, there are ways to do it, but it involves some backdoor methods.

Most folks, myself included suggest a KH for 50ppm or more, adding some bakign soda will raise it(KH).

If you are using Rainwater etc, it does not have any KH or GH, GH is the Ca and Mg nutrients in water, your plants will need this. You can mix with tap water to raise this up to about 3-5 degrees, 50-150ppm or so.

You can read www.thekrib.com for more on CO2 and various other sites, a good table is the pH/KH/CO2 table.

Measure your KH, then see what pH you need to achieve a CO2 level of 20-30ppm and keep the pH there during the day light times. Use only CO2 to do this, that is what the plants want, not acid "buffer" or peat.

If you need more CO2, add more CO2, seems simple but folks try to change their KH and pH a number of ways and start thinking...... the plants only care about the CO2. So add only that to change the pH to the level you are after.

Regards,
Tom Barr

Hard/soft and alkaline/acidic ared terms used to describe water. Often soft and acidic are used to describe acidic water while hard and alkaline are used to describe water with a pH above 7.0. The problem is that soft may not be acidic and hard may not be alkaline.

The pH scale describes the alkaline characteristics of water. Gh and PH describe the hardness of the measure of minerals in water. Generally, rain water will be pH neutral with nominal GH or KH.

KH is the measure of carbonate hardness or buffering ability of your water. Buffering abiltiy prevents substantial swings in pH. Plants in the aquarium contribute substantially to the diurnal fluctuation of respiration and photosynthesis that affects the oxygen and carbon dioxide levels that directly affect pH and hardness.

As plants receive light, they photosynthesize, using up the CO2 while releasing oxygen. Although repsiration is releasing CO2, photosynthesis uses up CO2 much more quickly causing CO2 levels in the water to fall and the amount of carbonic acid to fall. The release of oxygen binds organics and minerals together. With the falling carbonic acid, pH levels rise. When the dissolved CO2 is used up, plants find other sources of CO2 generally from carbonates (buffer or KH). As this is used up the surrounding water becomes softer and more alkaline (rising pH).

At night, photosynthesis stops and the plants respire CO2 decreasing oxygen. The dissolved CO2 binds with calcium increasing hardness and with organincs that lower pH by forming carbonic acid.

So, we have daily fl;uctuations in pH and hardness. Remember a chasnge of more than 1.0 in pH may cause health problems with some fish. CO2 injection with a lack of bicarbonates (KH) will cause the aquarium buffering capacity to drop that in turn creates larger fluctuations in pH levels.

Another caution is that the pH scale is logarithmic. A drop from a pH of 7 is 10 times more alkaline than a pH of 6. So, if you go from a pH of 7 to a pH of 5, you have gone from water that is 100 times more alkalintity (acidic).

The problem with many DIY (Do It Yourself) systems is that they do not adjust the flow of CO2 to the aquarium in consideration of the diurnal cycle or the buffer available to the aqaurium. DIY should never run overnight!

Automated systems should use a pH sensor to monitor aquarium conditions and in turn control a solenoid valve to regulate CO2 injection.

__________
"To steal ideas from one person is plagiarism; to steal from many is research."
researched from Steven Wright

wow, thanks for such detailed replies. I really appreciate it!
In my tank i have a lot of driftwood which i assume is why it is pH 6.0 (or below). The water being soft drops down easily to this level. Does adding bicarbonate of soda increase the hardness of the water, and will this cause a large change (if any) in my pH?
If it does increase hardness would this be reccommended in my case? I would like to avoid large swings in pH if i can. The DIY co2 is just a bit of an experiment atm but i have realised how little i know about water chemistry. I am happy to scrap the DIY co2 if it will hurt my fish. Please let me know what you think i should do.
Thanks again!

Hi,
The pH of rainwater depends upon what the rain falls through
what it is collected in, and how it is protected once it has
been collected.

In parts of the States, with a heavy industrial base, the
rain becomes very acidic and is called "Acid rain."

Once the rain is collected what is it stored in?
Some containers are made of materials that can leech
minerals into the water.
Some containers are open to the elements and dust/dirt
will find its way into the tank along with organics from
birds and bugs.

CO2 is not necessary for a planted tank.

It enhances plant growth.
It does this by exceeding the normal amount of
CO2 found in the water, and plants "think they are at a
banquet table" feasting on the carbon.

DIY CO2 is very hard to keep going and, IMO, it
is not "effective" in tanks larger than about a
30G capacity.
Larger tanks require more than two, 2 liter bottles
in series and bottles on the side ready to be mixed
to replace the active bottles.
In short, between the frequent system leaks,
bottle cleaning, and changing, it soon becomes a chore,
and many folks will "slip" in performing the
maintenance necessary to keep the gas flowing.

Once you mix up a batch, and have the gas connected, it
comes surging out at a high rate of delivery because all
the ingredients are fresh. As the mix ages the gas
production tapers off to nothing.
During the time of max gas production the pH will drop,
and then as the gas tapers off the pH will slowly
rise as less and less gas is produced (injected). At night,
with the lights off, and a new batch producing CO2, the
pH could drop more than 1.0 and stress the fish as it
cycles back up during the next day.
Unlike a bottled system, there is no way to regulate the
flow of gas. Once the ingredients are mixed and the bottle
sealed, you get what you get. To turn it off at night,
you would have to redirect the gas flow to the atmosphere
(unplug the hose) or insert airvalves. Then, when the lights
come back on, you would have to reconnect the hose(s) or
readjust the airvalves.

You can minimize the pH swings by having a KH of at least
3gdh. You can increase the KH (only) by adding bicarbonate
of soda. Be sure that you use only the unscented variety.
Add a teaspoon full, and let it mix for a few hours and then
test to see where you are, then add more if needed.
As was mentioned, a higher KH will buffer the pH swings and
minimize them.

Frank


Last edited by FRANK at 24-Jan-2005 09:45

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hey thanks,
i live in a semi rural area in Oz so the rainwater should be pretty clean (we drink it). The water i use for my fishtank is held in a closed plastic rain tank. The water that we drink is held in a closed, large concrete rain tank, which means that it is hard and very alkaline. I do water changes from the plastic tank so i don't get swings in pH and it stays soft and acidic for my amazon fish.
Well after all the advise i have decided to stop using co2 (unless if i buy a propper one in the distant future). Thanks to everyone who helped my through this.