CO2 and Algae

     We've had a few posts on the effects of CO2 on fish and plants lately 
     but nobody mentions the effects on algae (maybe it's too obvious?). By 
     mistake, I recently created a sudden reduction in the level of CO2 in 
     my tank from the top end of the range given in the "Optimum Aquarium" 
     table (or maybe a little above) to the bottom end of the range (or 
     maybe a little below). Result - no apparent effects on fish but the 
     water turned crystal clear (I never realised it was so cloudy) and 
     large quantities of filamentous green algae disappeared almost 
     overnight. I'm now trying to control CO2 at the bottom of the range 
     and I've added a pH meter to my Christmas present list!
     My questions are:
     1) Is this the obvious thing that everybody would expect to happen or 
     do I have an oddball tank?
     2) Can I expect that now I've got rid of the relatively benign green 
     thread algae, I can expect an attack from somthing really nasty?
     For those interested in the detail it's a 48" tank, Dennerle hardware 
     with manual CO2 control, kH ~ 4, GH ~ 6, phosphate unmeasurable, trace 
     of nitrate, daily PMDD and Mastergrow with the water changes.
     Jack Hardie in England (don't ask me about the weather - you don't 
     have time)