Any clues to why there is no pH drop, hence no CO2 being added to the
water?

Measure your KH, and use that to determine how much CO2 is in your water. With
Onyx as a substrate, you can't rely in pH changes to tell you your CO2 levels,
since it buffers the water.

Your tank is a bit on the large side, for a Carbo-Plus. Setting it at 4
probably won't be enough. Try 7 or 8 instead.

But first, get a KH test kit!


Leigh

http://www.fortunecity.com/lavender/halloween/881/