Scum On Water Surface In Drop Checker

[jimmy james]

I have a 18'x10'x10 open top tank iwagumi style with eleocharis acicularis, hemianthus callitrichoides, and some blyxa japonica, 18 ember tetras, approx 30 litres, the filter is an external eheim ecco 130 turning over 500lph, lighting is TMC aqua beam 500 duo reef white 8 hour period, pressurised co2 2bps 25ppm, temp 24 degrees c, ph 6.9, dose 1ml tropica nutrition plus daily, 50% water change weekly.
I'm getting a scum on the surface in the drop checker. Would it affect the co2 reading? And what could be causing it? I use a ph reagent that came with the drop checker from AE if that helps? I also have a nano tank with a drop checker and a blue liquid that came with the set up from dennerle and there is no scum in that one. As a matter of interest, when I run out of the chinese looking reagant that came with the drop checker, could i just use the ph reagant from my nutrafin kit? 5ml of tank water in the drop checker and 3 drops of the nutrafin ph reagant? Thanks.

 

[Coiln3107]

It sounds as if the problem is from within the drop checker. As the drop checker is in the tank inverted you cannot get surface scum or otherwise in to the trapped air in the mouth of the drop-checker as the checker is immersed. It sounds like a reaction from either the 4DKH water or the indicator solution. Try swapping one of them out and see what happens as one of them must be causing the problem I think.