Gang@ukaps During no lighting time the D.C on the right hand side of the tank looses its colour, the D.C on the left hand does not loose as much colour, I know the reason why the right hand D.C has a loss of colouration. The Co2 clicks into action two hours before the lights come on and both D.C are showing lime green at the point that the tank lights come on and are the same colouration when the lights go off - should there be any need for concern that the water column looses its residual Co2 thus showing a pale blue D.C. The left hand D.C does show a slight difference in its colouration but not as much as the right hand D.C. The funny thing is that there has been a very slight break out of green spot algea on the glass on the left hand side - this would indicate a lower flow / circulation than that on the right hand side, but there is more Co2 within the water column on the left hand side of the tank as the D.C is showing a grass green colour when compared with the D.C on the right hand side which is showing pale blue. Any thoughts Paul.