CO2 is actually very soluble in water (check Wetzel's book 'Limnology'). It is in fact at the same concentration in water as in air. The problem is its low diffusivity in water... about 10,000 time slower than in air. O2, on the other hand, has a low solubility in water AND a low diffusivity... this can cause problems if you dont have enough surface agitation. dave.