You can buy a pretty good size storage bin at Target, the dollar store, Lowe's for probably $5 if you hunt around
Small air pump
small sponge filter
Some some airline hose (some stores sell it by the foot)
And and a small heater
I think you can find all that stuff for $30
I really don't have a clear picture of the timeline or situation involved in this particular tank, or any information about the other tank you have, so it's difficult to say what happened to your fish
in the picture, the one fish I could see and, that was kind of upside down or sideways was a female.
Gudgeons hang out on the bottom, unless they're very comfortable and hungry then they will move up to the middle
(also if they are comfortable they will come up to you and interact)
They are shy and not as quick moving as Cory's. When I feed mine I have to make sure that I get food enough for them because they will get out competed
They move in little spurts and most likely she got extremely stressed out. Also, the decor in your tanks doesn't provide for enough room for the peacocks to sprint away, when they do move they move quickly and for quite a bit of distance.
Your decor is right in the middle which would have blocked them from their natural movement. she could have gotten injured along the line somewhere and wore out.
(Keep in mind the same scenario may occur with the remaining fish).
In addition, although the males are not aggressive when breeding, absent a place and atmosphere to breed may have placed stress on both fish.
If your water was clean and any other issues in the tank such as diseases parasites excetera are not present, I would say your fish did not die as a result of of acclimation. You do still have one remaining fish. In your first post you stated that you didn't really know how these fish act, but now you're saying the remaining fish is acting normal? Sorry, and I absolutely don't mean to be harsh, but I had to point that out.
I don't know much about Corys but I think that they can be together with other Cory's I'm not sure. You also didn't say what other type of fish you had in the salt and pepper Cory tank. Also, what you mean by overstocked? Perhaps they could go together and the gudgeon could go in the other tank? Especially if the other inhabitants are top dweller's and the water parameters are the same and the layout of the tank is suited for peacock gungeon.
Then then if you make enough room you could put the other new gungeon in there.
Getting the remaining fish a friend I do not believe is a good option for you right now and until you have the right environment to house peacock gudgeons you may want to hold off. The best option may be taking it back to the store for now.
I applaud your taste in fish by the way! They're wonderful fish, I have 7 breeding adults and probably 30 fry. I have posted a couple of videos and pictures of them here on the forum.
If you do take the time to prepare a right environment for them they are really easy to breed and I would get a few
This is just my take on your problem because I do have experience with these fish regarding their habitat requirements, temperament, and behavior.
As far as diagnosing what EXACTLY happened please take my input with a grain of salt since I really don't have enough information about your scenario