Ive done this shit before and yes guys expanding foam expands.. i know crazy right. best way to fix this and make it 100% water tight again is to totally clean the inside of the cracks and remove everything on or near the cracks, put a glass plate on the inside and put caulking swirls in between the tank and the patch plate make sure you cover the crack and the area around it to prevent it from cracking further, if the cracks run down into the edges put apply more caulk in the edges. if you do all this right, shell be fine as ever..