Roommate's friend's kid scratched the hell out of my tank. Must vent before I scream.

[Simons]

I you have warned the parents and the roommate of the dangers of using the magnet, which I have no reason to doubt that you did not; then the parents are on the hook for the entire bill. There is no wiggle room and no excuses, parents are responsible for the actions of their underage children.

I see this as not only very disrespectful that a parent would let their kid touch anything like that that wasn't theirs, but also completely and entirely on the hook for it's repair and whatever that would involve. I get pretty upset over this kind of stuff because IMHO it appears that a large number of parents just don't take it seriously enough.

While you are now stuck with 'living with it' as you say just lets the parents off the hook. I would be calling and telling them the seriousness of the issue and be prepared for a significant bill. I had a similar issue years ago (not with a fish tank) where my property was damaged by some kids after repeated warnings to the parents, they refused to pay for the damage so I filed a police report for vandalism. You should see how fast their attitude changed when the cops showed up at there door and they paid up or there kid was going to be charged.