This is somewhat of an old post but reading some of the stuff here I can see there's some confusion over the possible ramifications of infusing CO2 into the water in order to promote plant growth. Here's a little of what I know to help people's understanding.
Physiology: In the hemoglobin molecule, oxygen binds to the ferrous heme molecule for transportation. The affinity (strength of bonding) of the heme for the oxygen is labile and is reduced by elevated temperatures, organic phosphate ligands, decreasing pH and increasing CO2. As you can see, CO2 can decrease oxygen affinity in two ways: by dissolving and producing protons (the 'acid' component) which will decrease pH and thereby reduce O2 affinity via the Bohr effect, and more directly by reacting with the hemogloben protein. The latter is accomplished by CO2 reacting with -NH2 on proteins to form carbamino compounds. The amount of carbamino formation depends on the number of available terminal NH2 groups on the proteins. The hemogloben molecule is comprised of 4 globen proteins; 2 alpha subunits and 2 beta subunits. In mammals, birds and reptiles, all the terminal NH2 groups are available for carbamino formation, but in fish and amphibians the terminal NH2 group of the alpha subunits are acetylated and therefore not available for carbamino formation. Hence, all boiled down into more simplistic terms, CO2 is not going to affect O2 transportation in amphibians as much as it does mammals, birds, or reptiles.
Ecology: Fact of the matter is that most salamanders inhabit acidic environments anyhow. Whether it be terrestrial or aquatic, the vast majority of salamanders live in areas with lots of moss and other proton secreting plants, and associated with acidic conditions. They prefer water that is usually slightly acidic (particularly bog species) and frequently low in oxygen (due to high detritus and decomposition rates). The reduction in pH due to CO2 defusion is going to be rather small unless one is infusing extremely high amounts of CO2 and maintaining the aquarium under high pressure.
Mark does have a point of CO2 being heavier than O2, but this is usually only going to be a concern over land, not water. CO2 is highly dissolvable in water where O2 is not. The condition that typically results in hypercapnic waters (elevated CO2) is deeper waters have high CO2 low O2, surface water is the opposite. As long as circulation is maintained, you shouldn't develop a problem.