Do not place shims between a tank and a stand! They create pressure points which is exactly what you want to avoid
I agree with contacting the manufacturer and sending them a photo. I have read about quite a few tanks having this gap in the middle and I think that its ok as long as all 4 corners are supported but the manufacturer will be able to confirm that as I wouldn't take anyone else's word for it.
Great looking build btw, following along keenly.
Cheers,
Macca