Aquarium issue

[Tashoverride]

Hi all,
I'm having an issue with my tank at the moment.
A weird thick white line has appeared on the bottom of the tank and I am terrified that it's a crack.
I can 100% guarantee that the tank is level and whatever it is can't be felt on the inside.
The tank is my breeding tank, it's 130l glass set up. I contacted the store I bought it and they have basically said too bad.
And advice?