My reasons for thinking it is not heat is I do not see any blackening or discoloration. Also, one crack actually extends down the side of the tank about an inch. Could this crack be from an impact? I haven’t filled the tank to test it because I wonder if I should repair it first. The link to the repair story is broken, so I couldn’t read it. If there is a quick easy fix that may hold this tank together, I will do that first. There are four cracks in the top of the aquarium, they all run from a corner of the large cut out to the side of the aquarium. The two in the back run from the side cutout to the 1-1/4 diameter hole, to the back of the tank. One CONTINUES down the side of the tank about an inch, this portion of the crack is not perpendicular, but actually at an oblique angle to the tanks surface. The tank top has two rectangular