CO2 Injection

[James]

Ok in my quest to have a nice planted tank, I have a CO2 tank, plus items to use for injection.

So on a trail run, I hooked it up and started it in my 15gal tank. Now according to everything I have read, to have 30PPM CO2, it would have to drop the PH lvl about 1.0.
The tank started at 7.7, using Chucks Calculator that means I would bring it to 6.6PH to have the 30PPM. OK, since I have fry in the tank, set it to bubble slowly and over a course of ten hours, it dropped the PH to 6.4, over shot some, this is probably due to only some java moss in the tank. My question is, since I have shut off the CO2 in the tank, should the PH not slowly go back up to the 7.7PH, as the CO2 gassed off?

All my tanks are at about the same PH, and this one was tell I added the CO2. But after 24 hours, it has only come back to 6.6, I was thinking that without the CO2 it should go back to what I started with.

[chem]

The best method for measuring your co2 is with a co2 drop checker. They directly measure co2 concentration through the change of a standardized solution and indicator. You can buy them cheaply at a BA's or from people on plantedtank.

As for your direct question, 15g is not a large tank and therefore doesn't require a significant bubble count to acheive your 30ppm. In addition, your water out there is soft I believe so you do not have to overcome any buffereing capacity, meaning less co2 is required to elevate your ppm.

If you have little or no surface turbulance than your co2 levels can remain high. Add an airstone and your levels will drop quickly.

[James]

Thanks Chem

The best method for measuring your co2 is with a co2 drop checker. They directly measure co2 concentration through the change of a standardized solution and indicator. You can buy them cheaply at a BA's or from people on plantedtank.

More for your info, a drop checker does not mearsure CO2 concentration. They measure the PH againest a known KH reading. Its why most of the makers of these recommend using a solution of 4kh water rather than tank water and a standard PH test chemical.

I am aware that the tank size, more importantly the amount of water makes a difference in how much, why I had it set very low so as not to drop the PH to fast. Its not a change in PH that can stress out the inhabitants, but more, a fast change in PH can stress them.

I am more curious why the slow gas off in the tank, yes an air stone would help(not for air but for water movement) but the filter in the tank creates ample surface movement. CO2 tends to gas off rather quickly (I use CO2 at work exactly due to its gas off quality) as you know if you buy a drink at the movie, it goes flat rather fast due to the CO2 gasing off.

As I said, thanks, I was just thinking there is something else I am not looking at that is causing the slow gas off in the tank.

[chem]

The standardized solution in the drop checker means that variations in pH are a direct cause of CO2 conc. changes.

You also mentioned pH drops causing stress to your fish, it is actually changes in osmostic balance which creates the so called "pH shock" that people sometimes erroneously refer to.

The initial addition of CO2 into your tank will basically strip your water of hardness (Mg, Ca) before the pH decreases; that is the nature of buffering capacities. Because you have little in the way of buffering capacity left your pH will not change significantly. Do a water change.

[Soggybottom]

It has been a while since I injected CO2, but like chem said, I think it can stay in the water for several days if you don't have much agitation. If you want to get it out fast, I'd definitely throw an airstone in.
I think what happened was that I bought new lights three or four days after last injecting CO2. The plants went crazy pearling under the new lights for one day and then stopped, leading me to believe that there was still CO2 in the water and that they responded to it, with the response ending when the CO2 was used up. Not very scientific I know...
I think your pH should return to normal if the CO2 is all gone.