I'm wondering if CO2 saturated water is lighter than non-C02 saturated water.

Will it tend to rise to the top of a tank if circulation isn't sufficient to push it down? Or is this not an issue?

Why I'm asking:

I'm still trying to get good growth on the HC in my 180G tank. I'm wondering if it has something to do with the fact that it's lower than any other plant and maybe the CO2 rises away from it? I'm using AS so I know substrate is not an issue. It's been sitting in my tank for months with almost zero growth.

I remember reading that Tom had trouble getting HC to grow in the Esquire tank - maybe due to the depth? It seems that people growing HC have an easier time in smaller, shallower tanks.

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.

I've got good lighting on the tank, 495w total, but only 55 of it is directly over the HC, and the tank is 24" deep.

Would adding more light directly above the HC give it a boost?

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.

Assuming that this is true, part two of the question is:

Is Excel heavier than water, and does it make sense to add Excel as well as CO2 to supplement carbon for the plants at the bottom of the tank?

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.

I'm still a ways away from doing it critically.

But the question is rather interesting even though the "circulation/dissolving" statements elucidated thus far are correct I'd say.

But it's not why your HC is not growing, it's still a CO2 issue.

The client had issues because he kept meddling with the tank, then neglecting it for a period and then the CO2 would run out and he'd not check things for several days and there's about 8-9w/gal on the tank.

So that's not exactly a good case study
He's getting U gramifolia in place of it and has threatened to add Gloss(shudder)

Regards,
Tom Barr

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