Sterilizing tank and supplies after mysterious betta death...and maybe clues as to wh

Hi all,
Long story short, my daughter has a happy, healthy active betta, in a 5.5 gal, heated, filtered tank. Some silk, some live plants, fishy fort. So, she bought an identical setup and a new betta. Set up and ran the tank for 24 hrs, acclimated the new guy, put him in the tank. He was dead in less than 2 days. Went back to the not to be mentioned pet store and noticed that about 3/4 of the bettas that were there on Sunday are gone, and the ones left look truely horrendous. So, we are pretty sure we didn't kill him, unless with too much clean water after his horrible little cup. No sign of ich, velvet, dropsy. He had a pooched out belly when we got him, which shrank within the first few hours in his new tank. Ate well, but seemed to need to come up for air a lot more often than Pierre, her first betta. Seemed fine on Sunday, first day in the new tank, Monday he was clamped and unhappy, this morning after a water change seemed a bit less clamped and moving around more, midday was frantically racing to the surface, sinking if he quit moving, floating by mid afternoon. Ring any bells for anyone?

Question is, how do we clean the tank, plants, fort, to make sure that a new fishy doesn't get any funk ( if there is any) from the old one? And what about the live plants? Are they saveable, or do they need to go to the compost pile?

Thanks all,
Jennifer

Forgot to mention, we know the tank is not cycled after just running for 24 hours. That was to make sure water temp had stabilized and everything was working well. Daughter does 1/3 water changes every other day in her first tank, and had begun that same schedule in the new tank.

Jen

Jpappy,
if we want to bleach the tank, just many rinses and dry thoroughly when done? Would you get rid of the live plants? As far as transferring media from her healthy tank, it has a black foam filter and the little premade filter bag in the pump housing. Do we just squeeze the old foam onto the new foam, or maybe give the new tank the old filter bag, as it is almost due for a change anyway? She is running a bare bottom tank, so no substrate that might have goodies to share.

And, no, hadn't checked params yet. Tanks is still up, is it too late to check it now? I would like to know if it was an issue on our side before I possibly bring home some other poor fish and torture it
Thanks for the help!
Jen