I am very relieved that medication has begun and am keeping postive thoughts going your way.

Lowered oxygen levels are a bit less of a concern for bettas because they can go to the surface and breathe outside air like us due to their labyrinth system. As long as the fish is well enough to swim to the surface that is. Keep an eye on the gill movement and if the fish starts to breathe rapidly then you know there is either an issue of a meds reaction or oxygen depriviation issues. (Hopefully neither will happen.)

The others will advise about keeping the tank dark when medicating for velvet.

*sending good thoughts*

t