RE: CO2 in Pond

Hello,  I see no reason why you would need to add co2 to your outside 
pond.  I have substantial growth of plants tropical and other wise in 
my pond.  Without any additional anything  added, fertilizer or 
otherwise.  The fish add enough fertilizer.  Also remember that unless 
you live in a tropical area your typical warm water aquarium plants 
are going to die out over the winter and not return. IMHO.  FWIW.     
  Mike Bernardoni