Daniel Prince wrote in
:

My brother bought a shell at a shell shop (not an aquarium shop).
How do we clean it so that it will be safe to use in a goldfish
tank? Thank you in advance for all replies.

Depends on what kind of shell it is. If it is one of those, uh, scrolled
kind shells, you can’t see what there is inside and even if you dry it and
then cook it, parts of the original inhabitant might be there and start
rotting as soon as the shell is in the tank.

If it is a flat shell or just so slightly curved that you can be sure that
there is nothing unwanted inside, it can used. If it is a totally natural
shell, it starts decaying in fresh water, might turn ugly and greenish and
releases calcium and other stuff to water. Just very little, if tank water
is alkaline and hardish, but more if it is soft and acidic - and in that
case it turns the water better for a goldfish as they do best in quite
alkaline and hard water.

And like Nitesbane said, if it is painted or has been treated in any other
way, don’t use it. It is not from an aquarium shop so there is no guarantee
that it can be used underwater.

(I hope that your bro know what kind of water he has there, and I hope that
the tank is big enough for the goldfish as they get HUGE. Also I hope that
he knows that they are schooling and has more than one.)

Liisa