Well, if it is a deep enough scratch, it can cause a "run" in the glass, but that would have to be a serious scratch, that extended to one edge of the glass and then it would require some force. on a 30 gallon though, i wouldn't worry about it leading to a crack. It sounds like you're just starting off with this tank b/c i saw your other thread. I promise you that there will be plenty more scratches to come over the years. my 150 gal. has been running for 15 years now and it's got a fair share of scratches. And every single person with an acrylic tank (b/c they scratch so easily) will have multipel scratches on their tanks. there may be some super sensitive fish out there that doesn't like scratched tanks, but i've never heard of this, or perhaps have and just ignored it. my powder blue seems to love his scratched up 150. if it really bothers you, empty all the sand into a buck and get it polished out, but scratches are almost inevitable.