Heater Broke Inside Tank

By broke do you mean it shattered? or cracked? If theres pieces of glass everywhere then it might hurt fish who scavange at the bottom. Theres really not much you can do. Maybe use activated carbon to remove and dissolved chemicals or metals that may have leaked from the heater. I dunno if they use mercury in those heaters do they? I have no idea. Maybe call the manufacturer and ask them whats in there that may be harmful to your health.

You're lucky the fish didn't get electrocuted. I would say in most heaters not much to worry about (other than electricity), call the manufacturer.

Personally if it was my tank, i would partition off the area with the broken glass, then procede to do a clean up. Pick out the broken glass, and other heater components, possibly even lift the top layer of gravel off with a net thus removing any smaller glass fragments unseen by the eye.

You dont want bottom feeders skimming across glass!

It can never hurt to do a water change if your worried about tit though.