Rescued Peapuffer doesn't want to eat

Hey! So how things go, we got another rescue. We got two in the first place but one already didnt make it.

The Story: We got them from Aquantainces. We met two weeks ago, when we visited our friends. They told us proudly about going to buy peapuffers for their 200l. I adviced strongly against it. It's an 200l overstocked community tank with about 11 different species. A lot of them have long tails. Plus the tank is very bald, some low plants but nothing more. It just doesn't suit the puffers at all. Well they went and bought them either way.

On Thursday I got a call from them, the puffers were eating her fishes tails. What different could she feed them to make them stop. I explained that it's their nature. So she told me, "well then im going to flush them now"... I couldn't stop myself and told them we would take them in on sunday. Yesterday they showed up unannounced with a cup with the two puffers and a thank-you Baby ancistrus... As you can guess, I was very grateful for the present. -_- The puffers looked horrible. They had their Tail tugged, lying on the floor. Looking pretty skinny. They probably didnt get enough food in the big tank. The Ancistrus had a fast aclimatization in my big tank for now. Then I dripp aclimated them over 3 hours, as well as the cup allowed it. I was extra carefull because they didnt look good. Since I hadnt put up a tank yet, I put them in my 25l quarantine tank. There are only some shrimps and snails in. Both sank straight to the bottom, but then started moving, going up and down the glas continously. Always together, they seemed to get along well. Later at night when I last check one was already dead. The other one continued with the up and down.
The Ancistrus meanwhile showed to be very agressive and made me wonder of he beat up the puffers in the cup on the whole way to my place (about 30 minutes at least).
I'm really angry and feel at fault, because I should have done more or taken them earlier . My boyfriend already said its a lost cause with them and I did my best, but well.

Now: The left one, isnt eating and still going up and down the Glas. Although he sometimes circles around now. So far I put cattapa leaves in, and put watercare for new and stressed fishes in. I dimmed the lighting. I tried bloodworms and artemia frozen, as well as the snails ans shrimps in there.

Is there something I can do for him?