Various smaller crypts can do very well without CO2, in fact, some do worse or even disintegrate with CO2 injection. But they also require just low to medium light, high light will cause some of them to melt, inhibit growth, or simply covered by green spot algae as they're very slow grower; the general guideline is it will definitely not improve by CO2 injection. Do some search on "Cryptocoryne" on the net, or dig information from
www.tropica.com.
BTW, crypt family is VERY popular plants in Europe, and also appear very frequently in Amano's work. But its not popular in HK, Fat Txk only have a few spieces from Tropica (while Tropica have 16 sp. on their catalogue), and Tokxyo have about 8 spieces in stock. Try it out, crypts are facinating plants have hundreds if not thousands of sp., and all of their flowers are very beautiful (if grow hypotronic).