It’s a matter of pounds per square inch!
Usually these chips occur when a sharp 90 degree corner (very small surface area of glass) has a significant point pressure applied to it!
It’s all physics… but basically, a glass tank is heavy; it resists changing in motion, if it hits an object (especially a dense, immovable one, like the corner of a steel frame/etc), it’s a bit like you’ve just taken a hammer and tapped that sharp corner of the glass…
— basically, you end up with thousands of PSI on a TINY surface area of glass; subsequently making a spall chip! (Google “spalling” for more info on the phenomenon; it’s common in concrete, too!)
— a less brittle material usually handles this impact pressure, deforming instead of fracturing!
Here’s an excellent real-world example from my own research!
12mm (~1/2”) thick standard float glass — fine diamond abrasive blade (for glass/ceramic) on left, coarse diamond abrasive blade (for natural stone, porcelain) on right: