It is a charming idea though. This is the site, after all, where when faced with blistering hot summer days, miskairal began freezing drinking water bottles and floating them one at a time in larger tanks. ;)

What sized tank would this be used with. As you know, CO2 and strong lights (maybe 3-4 watts per gallon, 2 minimum) are usually used by people "water gardening" in larger (40-55-125-gallon ...) tanks. I might be willing to go to one of those larger tanks and slip an ounce or two of that seltzer water in the tank every day and just see what happens. Have the light(s) come on and go off at the exact same time every day. In fact, if you have even a very modest digital camera, take a photo every day at the same time. Start taking images a couple before you begin your experiment.

Shoot from the exact same spot at the same time everyday. If you can't shoot from a tripod (everyone else in the family wonders what those three pieces of masking take are doing on the rug) see if there is a spot on a table or from stool (oh no! more masking tape!) where you could take those shots. Leave the tank light, shoot without the flash and compare images after a couple of weeks.

But if the fish seem to be gasping at the surface or have trouble breathing, turn up the air turbulence and add a half a teaspoon of baking soda.

The other dicey thing about the carbonated water. If one adds too much, there could be a pH plunge and the fish could be pretty stressed. That is also the drawback to the DIY yeast generated CO2 generator. I have been tempted to try one of those a couple of times. (Who doesn't have 2-liter pop bottles around?)

About a year ago, yanketh submitted a fascinating series of images, which documented the changes that took place as his home made yeast based CO2 unit added a steady rate of Carbon dioxide to the aquarium. Notice also that he has a two-bottle sequence so that if the yeast and water over flow from the first bottle, that stuff settles in the second bottle. That has been a major drawback to the DIY units.

http://www.guppylog.com/story/2007/3/15/12447/7623

By the way if you are new here, don't be startled if an empty page comes up. Click on the reload button at the top of your browser until Guppylog comes up. <sigh> If no one is around you, your are allowed to yell all sorts of things until the page comes up. ;)

There is still a danger of over-loading CO2 but if instructions are followed and only a certain amount of yeast is put in there first bottle, it should be safer and cheaper that the seltzer water.

Here's a little something on testing, if you really want to get into it:
http://fins.actwin.com/aquatic-plants/month.200509/msg00902.html

By the bye, the CO2 line should feed into the tank in a way that the carbon dioxide gets spread around in a somewhat down ward direction. Yanketh fed into the outflow of his power filter. The gentleman with the more sophisticated CO2 bottle and feed had the CO2 line dispense the stuff just in front of his powerhead.

Here's an overview of CO2 diffusers:
http://athomasi.blogspot.com/2007/07/basics-of-co2-injection-for-planted.html

Also, no1likeme1414 posted a story here last August where he shared about his use of Hagen's Nutrafin CO2 injector, which is economical and easy to use because it runs on sugar, yeast and water (and something called "Stabalizer"). This might be what a lot of us would need and "only" costs from $27 to $45. Unfortunately he removed his images.

http://www.guppylog.com/story/2006/4/30/22742/2476

Here are several reviews o that system:
http://www.amazon.com/review/product/B00026058Y?showViewpoints=1

One can spend a hundred or two hundred dollars quite easily on valves, needle valves and a CO2 Cylinder. In fact one can spend considerable more than that on those systems. That makes the DYI two-bottle set-up or the Hagan CO2 injector sound like a much better idea - if you don't have little kids who would pull them apart. ;)

I am (or should be) really busy this afternoon, calling a US Fish and Wildlife Office, calling the repeatedly busy number of the airline (for two days) and trying to make space for the killifish which will be flown in from Netherlands next Tuesday for our Chicago Killifish show. There are a zillion things I need to do here, the car is getting tires and I need to run about 30 miles over to Joliet to borrow a case of eighteen 2.5-gallon Lustar tanks (with tops) so that I am sure to have room for those creatures until I can take them to the show site. So I've really not looked around the Net like I would like to.

Ha! While typing this, a call was returned from F&WL and the agent gave me a mini-course in importing fish. (Never lose my amateur status though.) And she had another phone number which allowed me to call up the airline and get disappointing answers to several questions.

One correspondent on plants has suggested that, "As long as you have a way of adding a known steady amount(Rate), you can find the CO2 injection amount you need. I believe this is why many folks in Asia and Amano (the author of the plant garden books and articles) use bubble rates, they work with this method as long as you add a steady amount." 20 or 30 small bubble per minute may be about right.

Technically you don't want your CO2 to go above 25 PPM in the water because if the tank hits 30 PPM the fish could be in real trouble.

One fellow felt that, "Plants and fish are still the best indicators one person has noted." I guess if they look good, all is well.

I wonder if a pH meter (or at least testing strips) is necessary. There could be a point of no return if the pH keeps dropping. By the way, at 1.5 or 2 degrees drop in pH, I start dropping 1/4 teaspoon of baking soda in the powerhead's or the power filter's outflow. ;)

Especially in a tank with plants that pearl in the day (they are releasing oxygen bubbles) the pH may vary as much as a degree over the day. Making up figures to illustrate this, the pH could be 6.5 at night and as the plants use up the the carbon dioxide (which is held in the water as carbonic acid) the pH may rise to 7.5 in the day.

The fear of a pH plunge at night is why some people turn off their CO2 units in the evening and turn them on when first up in the AM. In the dark the plants might use a tiny bit of CO2 - the fish provide that. But the plants in the course of respiration and growth will actually use more oxygen!

At a NANFA event a couple of years ago, I purchased a German unit which diffused CO2 from a solid piece of carbon. I've been meaning to try that. Let me get through with this fish show (a.k.a. my current albatross) and maybe there will be time to play with it.

This is more grist for the mill than a definitive response. I'm pretty sure that G ma is careful and patient enough to watch the tank carefully if she experiments with the seltzer water and could try it. If you don't have the time or patience, "Don't try that at home!" ;)

If you want to go searching with more questions, for starters search at:
http://fins.actwin.com/

G ma, if you do try that please let us know what happened.