I think your Vuppa plays a bug role in high oxygen levels. I have a modified filter guard in front of my sump overflow which I think may have increased the surface film. I never find the dead bodies with the amount of hardscape I have. I remineralize my water everytime with a water change (maybe once every 2-3 months as I'm dosing PPS-Pro). I should have a GH of around 5-6.
Vuppa plays a role because it removes the surface film
. I'd check the overflow to make sure it's actually skimming the surface. It might not be. If you're running CO2 and any of it reaches the surface, check to see if any bubbles are staying. If there are, you have surface film. If you can't tell that way then drop a paper towel on the top of the tank and angle yourself so that you're looking up at the surface of the water. If it appears any clearer where you took the paper towel off, you have surface film. Barr has a relatively large post about surface film and the effects of it on oxygen levels in an aquarium. Sloshing water or more exposed water in your wet/dry may be good for oxygenation but if you have surface film in the tank itself then it's kinda a moot point.