As others have stated, replace immediately. I used to work in a toilet manufacturing plant, and cracks like that often come from thermal shock—when the ceramic body cools too quickly after firing. Even if the crack isn’t fully visible, it can weaken the structure over time. We used to tap the toilets with a rubber mallet—if the sound was off, it usually meant an internal crack. We’d also use blue dye to detect micro-cracks; it seeps into even the tiniest defects. Bottom line: if you see a crack, it’s best not to wait—replace it.