Does CO2-saturated water float?

but it IS growing? Its not fading away, right?
Then its probably more due to lightning not "reaching it" than your CO2 level.

 

Circulation is good. CO2 is always going to try and escape.

 

Using my non-scientist logic on this question: CO2 is freely able to move around in water it is dissolved in. It doesn't float, because it doesn't displace any water. However, at the water surface the CO2 is always leaving the water and reentering the water at the same time, with the process always favoring movement towards the substance having the lowest concentration of CO2, which will be the atmosphere. Thus, the surface layer will always tend to be depleted, causing a gradient to be built up in the tank, with the surface having the lowest concentration of CO2. If we have good water circulation it will only be the surface thin layer that will be low in CO2. But, the bottom of the tank, even with good water circulation will have a stagnant layer, because the circulation will always be laminar flow, not turbulent flow. That layer will get minimal water exchange with higher levels, and might well be short of CO2 as a result.

 

Remember that just because you see "bubbles" maybe coming out from your diffusor etc and floating to the top doesn't mean that the water isn't saturated with CO2. At the molecular level CO2 bonds are breaking and reforming with H2O, all invisible to the naked eye.

Since the fish tank is essentially a container with an open top the partial pressure law will have the CO2 exiting the top of the tank to mix with the atmosphere "degassing" etc. So towards your question, I would say that with a constant flow of CO2 into a diffusor all the plants in the tank are getting almost equal CO2. BUT, I would be curious myself to test as an experiment in an aquarium the levels of carbonic acid at the surface and at the bottom of an aquarium say 20" deep, I'd like to see some figures.

 

Tom is pursuing a membrane type drop checker head on a pH probe so he can look at CO2 levels all over the tank with a very short response time. That will be an interesting study when he gets it done. My bet is that CO2 levels in various parts of the tank vary all over the map from zero to 40+ ppm. And, I think good water circulation will reduce the spread of values a lot, but not reduce the spread entirely. I'm basing this on....ah....some very careful calculations using advanced calculus.

 

But CO2 will still want to equlibrate itself in all of the water. It might just take abit more time.

 

Here's a quick diagram of my tank & circulation system:

Sorry for the crudeness of the model. Also this is only the circulation/filtering, I'm not showing any electronics or heating etc...

As you can see my CO2 is entering from the side of the tank and flows right at my canister filter's return, which send current towards the front of the tank and back towards the inlet of the canister and CO2 pump. Because my tank is long I added a Magnum filter to the right side to give some current flow from that end toward the canister. I only run a mesh filter inside the Magnum and clean it out weekly.

Anyway don't seem to have (that I know of) any issue with the plants in all areas of the tank getting CO2. I was thinking about splitting the return on my reactor into 2 or 3 outlets instead of one, like one on the right, middle, left. But not sure if the effort would make any difference without knowing if there is say less CO2 on the bottom right side of my tank than the left.

Tom/others any comments on my current circulation setup, room for improvements?

Thanks,
Lee