dkfennell

I agree with guaiac_boy on this. If you look at hoppy's post, you will see that he is quite candid that the color chart he was using to measure pH in his distilled water was hard to distinguish. And if you do the calculations you will see that even a 0.1 difference on a log scale makes the result widely different.

The problems are: 1) pH tests are not reliable (at least they can't be replicated by different people using color tests; and even if digital probes are used, you have to have careful calibration); 2) you can't figure out what acids are in aquarium water, or at least you can't control for them; 3) KH can only be replicated if people "calibrate" the test with the method Tom Barr has repeatedly suggested.

There are probably other points, but my brain gives out when I keep going round and round on this ppm of CO2 thing.

In any event, the foregoing is enough to show that we cannot figure out the ppm of CO2 in the tank with any where near reasonable specificity. And I for one will not take any action based on someone else's statement that "so-and-so happens at x ppm CO2" or "the fish can survive even in x ppm CO2." So the question is: what is a reasonable way to describe how much CO2 I put in the tank (either to someone else or in my journal, for future reference.) And I think the answer is: enough CO2 to reduce de-gassed tank water from pH __ to pH __, as guaiac_boy has often suggested.

By the way, I think you should not only specify the pH drop, but also the beginning and ending point. If you go back through this thread and the links contained in it, you will see that under the formula shown by both MarcosB and Roger Miller, the amount of CO2 injected in the tank which results in a pH drop of 1, varies (by exponential factor) depending on the value of the starting (degassed) pH.

Darrell

RTR

Air equilibrium for dissolved CO2 is not a hard, fixed number. It depends on the elevation above sea level and the atmospheric pressure.

R0bert

And one person's 1 ph drop could be half another person's 1 ph drop.

Using a cheap digital ph probe, my degassed tank water is ~ 8.5 and my tank is ~ 7.0. That's a one and a half ph drop and yet I still have algae issues.
Including green spot where P is high - ie a CO2 issue.

My ph tester may not be accurate, and Hoppy's may be worse, so still no one knows if the starting point is 0.5 or 3 or 4ppm. (According to the chart, 8.5 ph at 12 kh = 1.15 ppm CO2.)

I'm just guessing, but the KH might have something to do with the equilibrium value. Mine is 12. So maybe as well as describing our "drop" in ph, we shouldn't neglect to state our KHs and hence the theoretical ppm based on the old chart.

BryceM

Bottom line: I think we can all agree now that the exact concentration of CO2 in a person's aquarium is pretty hard to know precisely. It probably isn't that meaningful when a person states that his/her CO2 is X ppm. There are significant errors in our ability to measure it, especially if you acknowledge the possible compounding effect of measuring mulitple variables with imprecise methods. There are probably significant errors in our understanding of the interactions of CO2 in a complex solution containing a mixture of many acids, organics, and buffers.

There are many issues that come into play when trying to defeat algae - CO2 is just one, albeit perhaps the most important. If there was a universal solution, someone would have figured it out a long time ago, and we'd all be following it now. There is no sense in being truly dogmatic about most anything we do since there are few things that we understand well enough.

It is perfectly reasonable to use as much CO2 as your fish can safely tollerate. Increase it until they show stress and back off a couple tenths. You need to be reasonably certain that your KH is stable if you are making decisions based on pH readings.

It seems reasonable that one could trust a relatively new, recently calibrated pH probe with more confidence than a bunch of hobby-grade KH/CO2/pH kit results. In the end, we're not trying to put a man on Mars or keep a patient alive in the ICU. This is a hobby, and there are plenty of other things to worry about too.

I'm getting the sense that the usefulness of this thread has just about run its course. If someone can find actual evidence that supports a better way to do this, please share it. I'd especially be interested in knowing if someone can accurately determine the resting equilibrium of CO2 in a buffered solution, or if they can determine if the actual KH affects the degassed CO2 concentration.

R0bert

Me too. It would be nice if someone who has access to quality ph and kh testers and pure water and buffers can publish a set of results for us to look at.

I agree that as part of a hobby we shouldn't get hung up on exact ppms etc, but it would be nice to have a set of standard resultsto compare with.

SnakeIce

This idea sounds good, however a few things need to be spelled out so the brand new planted aquarium keeper that has no history of dealing with measurements of water hardness and ph can use this.

One caveat mentioned is that your buffering/hardness needs to be at least a value. I have run into several units of measureing for this; grains of hardness, mg/L, parts per million, and any others I've forgotten. For simple use a target value range listed useing all the units of measurement that various tests may return and an amount of ___*__ per unit of volume to add to get at least the target value if your buffering falls below the target.

*Probably would be fine to list how much to add to bring distilled/ro water to that level since higher than the target is not a problem.

A mention of both comercial products and chemical names/symbols of what can be used to raise your buffering would help as well.

the various units of measurement and not haveing conversion table for the target level of buffering has been a trouble point for me. Once this is explained, the chart in this thread would be easy to use. I think this idea with these explanations is a step towards developing a formula for success.

hoppycalif

I like this method best of all, providing we can be reasonably sure that none of us will have enough CO2 to harm the fish if we use it. When I check the CO2 I look for .8 to 1.1 drop in PH before I consider adjusting bubble rate. So far I see no gasping fish when I am at either number. And, I am reasonably sure I am at an adequate CO2 level for either number. Frankly, I doubt if I could recheck PH ten times, with the PH above 7.0 or so, and get the same number more than half the time unless I fudge my eyeballs while doing so. (Don't try fudging your eyeballs unless you are a professional!!)