I think it's fine mine have that and they are new if I'm seeing it correctly. It just looks like a textured bottom (my 6.5 gallon doesn't have it but it's a different brand). It it is an actual crack and you can see it go through then obviously stay away from it.
*** disregard I think I might...
They will add an extra coupler (for a fee) on there for you so you can just buy a thermometer to screw in there. Better have them do it no than you try later. I just ordered mine, cant wait!
If I had this cleaner o how much keener i'd be
For no more would wash-day bring poor demeanor.
Smile and laughter would accompany dirt-busting
As I wash away the disgusting and encrusting filth.
I ask for this gift not for me, no
But rather for the old crow
So that finally she may stop...