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It is carbon dioxide.
It's ok, everyone's mind goes blank sometimes... that's what friends are for... lol
To elaborate CO2 (carbon dioxide) is emitted from live animals like your fish. So they breathe out CO2 just like we do. Plants on the other hand have 2 processes, the first when lights are on, they consume CO2 and produce O2(oxygen). Then at night during lights off, they consume O2 and respire CO2 back into the water. In respect to aquariums, this is important mainly in planted tanks. Some people inject CO2 or use a liquid CO2 supplement (like Seachem Excel) to make more CO2 available for plants which helps them out compete algae. You can also over do this and put too much CO2 into the water, either by overstocking with very little CO2/O2 air exchange into to water or by too much CO2 injection. This can essentially suffocate your fish, and limits the O2 amount in the water.
What's really interesting about Excel is that it is not CO2 at all (you can't have a liquid form of the gaseous molecule), but rather a product that allows plants to bypass the involvement of CO2 in the photosynthetic process. It's a liquid means of allowing plants to have available carbon without the hassle of a CO2 setup. Interestingly, Seachem (makers of Excel the "liquid" CO2) has this to say: "Flourish Excel provides the same benefit as CO2, i.e. it provides the plants with a source of carbon for growth just as CO2 provides them with carbon. However CO2 by itself will give you quantitatively more growth than Excel by itself, although Excel does provide a substantial amount in comparison. If CO2 is a 10, Excel is a 6-7." http://www.seachem.com/support/forums/showthread.php?t=3595 edit: the "Quantitatively more growth" is often undesirable in most setups with most plants unless you really enjoy pruning. I use it, but only because it allows my plants to "bubble" and emit more oxygen, which is fun to watch.
Good stuff Phishphin. Real CO2 gas injection is a bit better for pure plant growth. The combo of using both can be even better. But I also like the secondary use of Excel as an algaecide. Aside from helping the plants, you can also load up a dose (based on your size tank) into a syringe, turn off your filter and air pumps to get the water really still, and squirt directly onto problem algae spots. Let this sit for 15-20 minutes then turn everything back on. The filter and water movement will then disperse the Excel around the tank for plants to use, but this high power dose will kill the algae. If you have a lot of algae, you can get a new spot every day over the course of a week or so and you will have killed most of the algae. Of course, figuring out and dealing with the cause of the algae is always best so it doesn't come back.