Question on Oxygen levels + filter

[swozzie]

I have an O2 measurement kit that measures in mg/l but I don't know what is good and what is bad. Can anyone help?

Also, I have a well planted tank (using deponit/sliver sand/inert (1.2-2mm) gravel) with an undergravel heater cable that uses convection to move water through the substrate. Presumably my biological filter exists in both the filter and the substrate but which is most effective or most important of the 2 biological filters?

 

[tstenback]

I dont know if This may help with the o2 question. As for the two biological filters (substrate and filter) the filter would be most important as your water flows through the filter 4+ times an hour therefore it provides the most surface contact between bacteria and the water. HTH