My second (current) Green Spotted Puffer.
I loved the first one (named "Rasputin"), but he had a ton of internal parasites and died after months of off and on treatment. I'd get him to pass several worms, keep up treatment for a few more days, nothing else would come out and I'd quit. He'd be fine for a month, then start with the constantly sunken belly thing again. I was feeding frozen bloodworms (Hikari), freeze-dried krill (Tetra), and tiny snails I got from PetSmart for free on occasion. I spent so much money on him, but he was a great fish, always excited to see me, very curious and interactive. The last round of treatment tons and tons of worms came out of his gills and he died, probably from asphyxiation. I was devestated and left the tank fallow for at least six months.
Finally, tired of looking at an empty tank I got my current GSP, "Frumious Bandersnatch". She is healthy as anything, living a little over a year with me now incident free. So why don't I like her? She ignores me 99% of the time, just paces up and down the glass day in day out despite having tons of decorations and such to look through and bits of food occasionally hidden to find (honestly I'm trying, I know they need things to do) and refuses to eat anything but freeze dried krill and the occasional bloodworm. All the snails I've put in I've had to fish back out a day or two later when the high salinity in her tank kills them. She let 5 of the last 25 ghost shrimp I fed her live, which is nice only because they help clean up all the food she ignores. I've tried feeding ghost shrimp since and she has no interest in them. Also tried bloodworms, tubifex, blackworms, and more. She'll eat a few, then let the rest sit on the bottom for a day till I suck them up so they don't rot.
I tried putting another GSP in with her in hopes it would liven her up and reduce the pacing, but she just freaked out for about 3 hours while it followed her around, shooting all around the tank and hitting the top several times, then turned around and removed half its fins in about 2 minutes. I pulled him out for his own good and returned him to the local fish store, and no tankmates have been attempted since...
I guess she's not a "bad" fish, I just really liked the personality of the first GSP, and she's comparatively boring and unobservant. Yet, as many others have said about their "problem" fish I can't bring myself to bring her back to the store or otherwise re-home her, so I'm "stuck" with her.
Also, the Spotted Climbing Perch I recently put in my 29G freshwater tank as a sort of centerpiece fish is taking my Neons out one by one every few days, but I think he's getting used to eating flake so hopefully that won't keep being a problem. He's cool enough that I honestly almost don't mind. If I could just "train" him to eat feeder guppies and leave the neons alone that'd be great, but I suspect I'd just end up with a tank full of feeder guppies. I'm a softy. "I can't let him eat you! You have pretty colors!"