Hardiest Tetra, had to be a couple of White Skirts I had back in middle school. I didn't know what the heck I was doing, since I was in the 6th grade and had no internet. Those guys handled water straight from the tap like some sort of fish version of Chuck Norris.
Hardiest fish ever, however, has to be Bob my platy. One of the first fish I bought this year, when I took up fish-keeping again. Right out of Walmart into a 1 gallon tank with a molly, then into an uncycled 10 gallon, before I knew otherwise. Handled it like a boss, she did. (Yes, Bob's a girl.) After a month and a half, she got fin rot after one of the other females died, leaving her alone with the male, Skittles. She was bit up pretty badly. Put her in a quarantine tank, gave her some Jungle Lifeguard. She got better, I put her back into the main tank, where she was well for another week, then got rot again. So I went to Walmart again, (I am a poor college student, I practically live at Walmart... Don't judge me!!
) and bought some Malefix. Bob is now back in quarantine, but recovering nicely once again.
Bob is the Chuck Norris of all Live-bearers!
Ok, all done...sort of went really off-topic on that one XD