I know other plugins have this issue, unfortunately. The screen is based on the resolution so it may not be positioned correctly. My feedback is to find a placement for it that has breathing room to move around within acceptable areas for different users.
As for testing, I decided not to implement due to the visual issue plus I noticed it wasn't working during my testing. I'd go underwater, see the blue bar but it wasn't going down. I then confirmed my tank was using oxygen. It's possible that I reloaded the plugin while underwater. Perhaps you might want to reset any measurements when the plugin is unloaded.