Ok, it seems that the algae going white on the screen is nothing to do with Co2 additions. I have been feeding the screen pump with air from outside the sump area. This may well be wasting Co2 that is respired by the tank inhabitants at night. As I see it, the Co2 will travels downwards to the sump and lie on the sump surface until it gets disturbed by opening the doors on the cabinet etc, or just overflows out the sump into the house. I'm gonna utilise this extra Co2 by shortening my pump air feed to below the sump lip, so it sucks in the air from the sump surface. It's worth seeing what happens as it don't cost anything having the skimmer air inlet at the surface may serve a similar function.