Not Good!

[Dale]

Hi Jaws,
It looks like your wave maker was mounted or resting on that part of the tank and the flexing caused the crack (from what I can see) The slot for the overflow creates a short piece of glass there which would be weaker than if there had been no overflow slot.

It's totally up to you to balance cost of replacement vs the asthetics of a visible defect vs peace of mind but I can say I've used several tanks with cracks in them like that for many years without difficulty.

If you decide to use the tank you should find a rectangular piece of glass that will cover the crack liberally. Dry and clean both surfaces and smear silicone thinly on one side of the "patch" glass and stick em together. Oddly enough, that part of the tank will be stronger than before it broke.

I'll try to edit in a picture to illustrate.