An airstone will reduce the O2 by turning it on at night as the water has been oversaturated(beyond 100%) with dissolved O2 from the plants all day.

Why remove it?

Wait till the light have been off for several hours, maybe 6 or more.

Depends on your plant growth rate and density, but I routinely have 140-150% when the lights go off.

In the morning, I have 90-110%.

Regards,
Tom Barr