How to calculate my ACTUAL CO2ppm

[fish_4_all]

Ok, I have plant problems that will not go away and all things point to CO2 being too low. The problem is it tests out regularly at 53-60ppm. Both tanks read pH 6.6 and KH 7-8. This only changes 6-12 hours after a water change and refreshing my DIY mixture.

The question is, how do I test my CO2 so I know what the actual readings are so I can finally get rid of this BBA that looks thicker than the hair on my head at times!

 

[czcz]

I realize this doesn't answer your question, but since it is difficult to get actual CO2 ppm, one method is to take the BBA as a sign of not enough CO2. Gasping fish means too much CO2. Keep it simple. Somewhere in-between is often the most reasonable target, especially with DIY.

 

[7Enigma]

The easiest way to determine actual CO2 concentration or more accurately to determine whether other factors are throwing off your values is to take tank water and let it stand overnight. Then take KH and pH measurements. Here you can see where problems might arise. If your tank water that was set out gives you a higher CO2 level than 5ppm you can be pretty confident there is something buffering the tank, OR your test kits are faulty/improper testing.

Also tank current plays a large role in deficiency symptoms. I was seeing all sorts of algae and plant deficiencies on one side of my tank even though all of my levels seems fine. I then tested directly from that area and noticed it must be a low flow area since some nutrients were lower than my normal testing.

This also brings up a point I constantly harp on: pH measurements are the most variable test we can do since the testing procedure has an inherent flaw.....unlike other nutrient tests the pH changes constantly. The more manipulation you do prior to reading the more change is present. That's why I use a pipet to draw tank water from the middle of the tank. Normally your pH will be slightly higher near the surface since the CO2 is gassing off than near the substrate. Every second you wait or container you transfer prior to measuring will be gassing off CO2 and increasing your pH reading. Fortunately this usually means that the pH measurements we take err on the side of too high (ie they are in fact lower than what we measure), but in your case its possible there is some other factor that might be throwing off the pH.

Done any hydrogen peroxide treatments recently?

 

[malkore]

yes, take a rested tap water measurement first. if the resulting CO2 level is under 4ppm, then your water is 'normal' and CO2 calculations will be accurate within a couple ppm...good enough for our needs.

 

[fish_4_all]

I test my tap once a month and it always read pretty much the same. pH 7.0 and KH<1. I let it sit for 3 days just to make sure it isn't gassing off CO2 still and has equilized.

I will let tank water set and see what my reading come up with. I kow there is going tobe some KH readings because I have some shells from snails and some in my gravel that raise it slightly. Other thanthat I have never done anything to change the KH of my tanks. Right after a water change my pH goes to 6.8 and KH drops to 6, both changes cut in about half. Is a drop of 4 dKH normal? This would leaveme thinking that my tank has a normal KH of about 2-3.

Normal readings:
pH 6.6
dKH 8
pwc gives 6.8 and 6 so water is nuetral from a chemistry ascpect and results in .2 change of the total 4 ppH change from tap and with a KH going form 8 to 6, this means that the tank KH should be around 4 on a normal basis.

Theoretical pH would be 7.0
tank pH + 2 times the change after 50% water change
6.6 + (2*0.2) = 7.0

Tank mean KH = 8 - 2x Kh change after 50% water change
(8 - 2*2) = 4

We will see how close it actually is after I let some tank water sit for the rest of the day

 

[Purrbox]

Looks like there aren't any acids/buffers in the tap water affecting the KH/pH relationship. As long as you haven't introduced anything into the tank water that would affect that relationship your readings should be fairly accurate. Your next reading will tell us for sure if this is the case.

 

[fish_4_all]

Ok, I don't know what is going on but my tanks just went haywire.

Tank #1 Set out water pH 7.2, dKH 7
Tank water pH 7.0, dKH 8

Tank #2 Set out water pH 7.2, dKH 9
Tank water pH 7.0 dKH 10

Disregarding the fact these number are way off from my normal pH of 6.6, what does this tell me?

 

[Purrbox]

This tells you that there is a buffer present in your tank water that is throwing off the KH/pH relationship.

Tank 1 Rested: 13ppm
Tank 1: 24ppm
Tank 2 Rested: 17ppm
Tank 2: 30ppm

Since both tanks are showing CO2 levels in the teens when they should be at 3ppm after being set out, your actual CO2 levels are much lower than you want them to be. You'll need to use this alternate method for calculating your CO2 levels.

 

[fish_4_all]

Ok, I must be stupid because the only thing I get out of the link is that my optimul pH is 6.8. Man I hate chemistry! Except for blowing things up of course.

 

[Steve Hampton]

I think I found the problem. See this link.

http://www.aquariumadvice.com/viewtopic.php?t=87462

 

[Purrbox]

Basically that method allows you to determine what your PH should be based on two samples. One measures a sample of water that has been sitting for 24hrs to gas out all the CO2 (3ppm) and the other measures a sample of water which you have been blowing bubbles into with a straw bringing the levels to 60ppm. Then you use a formula to determine your target PH for CO2 = 30ppm, which would be slightly different than the one in the article. The resulting PH is the level that you would want to keep your PH under to ensure good CO2 levels.

Target PH = (High PH + Low PH) / 2

However, since Steve Hampton has identified the buffer that is skewing your results, it would be better to deal with it that way instead.

 

[fish_4_all]

I also forgot to add that the readingsI posted were done after mixing the tank very, VERY well. This has given me accurate over all tank readings and I think a lot of my porblems with my testing is I have been testing from the wrong places and not mixing my tanks. This also says I need to get a powerhead in my tanks as soon as possible. One for circulation and one for the benefit of better CO2 diffusion. Will see what happens.