A few months ago, all three of my friend's goldfish mysteriously died.
Their water levels were ok, and they all went within a few days of
each other. My friend considers himself a very responsible pet owner
and took it pretty hard, and gave up on fish.

I now have his 10 gallon tank, complete with light, filter, heater,
and other cool stuff. Since we don't know what killed the old fish,
what sort of precautions do I need to take? Everything has been dry
for the past few months, and I just finished cleaning everything off
with scalding hot water and a new sponge. Is that enough? I also have
various anti-fungal and anti-algae tablets that he threw in with the
rest of the stuff. Is there something specific I should treat the
water with (in addition to the chlorine remover)?

I plan to set everything up and let it run for a few days to make sure
everything is running smoothly. Once everything's up to snuff, Bob the
Oranda will have a new home