Received a damaged aquarium and going to replace it with new one but was wondering if this chip/ crack renders the tank useless. It's not on the main tank but near the overflow
As long as it's inside the tank, it should ok.
It looks to me like it's on the inside overflow box.
If it's glass a little silicone on the inside of the box where you can't see it, will keep water from seeping in.
If it's acrylic, a little weld-on #16.