[JoelB]

A few random questions, please, as I'm thinking of doing fixed CO2 and the oxygen levels concern me.

If I inject CO2, how can I maintain oxygen without disturbing the surface (or injecting air) and wasting the CO2?

Should I be concerned about activity like cycling or algae outbreaks, that the oxygen doesn't get depleted?

Will an airstone in a NPT maintain both (relatively) constant O and CO2 levels?

and

How can I measure oxygen levels without waiting for the fish to visibly suffer?

 

[Hoppy]

Most of the oxygen in a planted tank comes from absorption through the water surface, so the more surface area the easier it is to maintain the concentration high enough. The best way to get a big surface area is to have the surface almost covered with ripples, from water flow just below the surface - not splashing, just rippling. This will also deplete some of the CO2, but CO2 is very cheap, so just raise the bubble rate to compensate.

Measuring the O2 level in the water isn't necessary. If the water surface is rippled you will have plenty of O2.

Most NPT's don't use CO2, so an airstone is acceptable. In an NPT the CO2 comes mostly from natural decomposition in the substrate.