So I picked up four new 40 breeders during the recent PetCo $1 per gallon sale.
Placed them on a previously used, well built, perfectly square wooden rack.
The last one I put in place wobbles like a drunken monkey

So I slide the one next to it over in the same spot...perfectly fine.
Then I take the questionable tank and check it all the way around with my square
and with several levels, etc.
It is literally out of square right from the factory

Looks like I'll be swapping that one out tomorrow at PetCo.