New ADFs died overnight

[BlueSky99]

I decided to buy 3 african dwarf frogs yesterday from my LFS to eventually add in with my (now) alone one. These guys' bodies are about 3/4" or smaller and they're all skinny, so they're young. They seemed fine when I put them in the quarantine tank, were swimming around, and two kept going behind the HOB filter because that's where the least amount of current is (ended up having to baffle the filter). Earlier this morning I noticed they were all in the same area, floating on some leaves of the silk plant. I thought they were just hanging out as these guys do, but I just moved the leaves to make sure they're alive and they're all dead. I don't know if it was stress or a disease that killed all three within 17hrs of them being in the QT (they were probably in the bag for 1.5-2hrs). I haven't tested the water because I just set up the QT when I got home with them, but the temperature is 76F. Their skin doesn't look shriveled or light, in fact it looks like an ADF that just shed (aka healthy). Two of them happened to turn upside down and there's a red area kind of in the direct middle of their belly.

I don't want my adult ADF to be alone so I want new ones but I'm not sure where to go from here?

edit- Oh, and I let them acclimate in the water for about 30 minutes, then added about 2 oz. of the clean water to their bag every 10-15 minutes about 3 times, poured out half the water, then added 2 oz. of the clean water about 3-4 more times over ~30 minutes. Then I poured out most of the water and slowly poured the rest, with the ADFs, into the QT.

[craig121]

Hi There's been no mention of water conditioners these guys are very fragile especially young and with water contaminants ie Chlorine metals etc! What did the last batch die with how many & how long ago