Fantail Gold Fish

I recently bought a new tank for 2 fantail goldfish because their
original tank looked way too small (the gf brought them to our new
house from her parents) and i've put a fuval2 with no heater in. I
don't know anything about these fish appart from they seem very hardy.
I've got some tropical fish so i've got all the test kits which i'm
using on a regular basis on the fantails.

A few questions, how long do they live? What sort of conditions do
they ideally need, i.e. tank size (the fish are about 3-4 inchs long.
Can i put any plants in the tank. I'm also feeding them normal
goldfish flake food. Do they like to eat anything else?

Thanks