my 75G diddnt come with a stand and I am using my fireplace as one, unfortunatly it sticks out 1 and 3/4ths of an inch over, will this cause a problem?
If it's a glass tank and the edge on the front isn't resting on the fireplace it will put stress on the seams and could cause problems like cracking/leaks/75 gallons of water on your floor...
Is it on the floor of the fireplace, or the mantle? I'm 100% sure a mantle would not have been designed to hold 600 pounds which is about what that tank weighs filled. You really should find a stand.