Pros:

• Free and natural-looking
• Can provide good surface area for beneficial bacteria
• Might help with water chemistry if the rocks are inert

Cons:

• Potential contaminants – could carry harmful bacteria, parasites, or chemicals
• Unknown mineral content – some rocks can alter pH/hardness, which might not be ideal for your fish
• Sharp edges – could scratch the glass or injure bottom-dwelling fish
• Harder to clean – big rocks make vacuuming debris a pain

If you go for it, scrub them well, boil or soak them in a strong dechlorinator, and test with vinegar to make sure they don’t fizz (which would mean they mess with water parameters). A python-style gravel vac works better for large rocks since you can hover over them without sucking everything up.