I bought a bottle of Seachem's Flourish Excel - apparently it's basically glutaraldehyde, which serves as a source of bioavailable carbon for aquatic plants (but not algae). It's also supposed to function as an algaecide, especially at higher concentrations, although it isn't actually marketed as such (see Seachem's FAQ; there's also a lot of talk about this on discussion fora).
While I'm terribly curious about the mechanism of action of glutaraldehyde (or whatever it may break down into), I'm mostly curious about whether the plants in my main tank are CO2 limited. My light levels are well below what's recommended, but when people talk about planted tanks, they talk about light and carbon dioxide. Given my current light levels, would my plants be able to utilise additional carbon? I hope to find out.