Cracked tank?

[Delphinus]

^^ what he said. Don't despair though, it might be a scratch. I have a nasty scratch on the back of one of my tanks and the way the light catches it I thought it was a crack at first. Can you post a picture of it?

If it is a crack, then the panel needs to come out and be replaced. A tank builder worth his salt should be able to do this for you (Bow Valley used to do this, don't know if they still do), or if you're energetic enough you could do it yourself...