I have a well established 55 gallon tank that has been up and running for years. There was a mismash of fish, including 1 green cory cat, 1 columbian tetra, 1 penguin tetra, 2 long finned red tetras, and 5 red eye tetras. I purchased 3 fancy male guppies, 6 female guppies, 3 long finned red tetras and 4 cory cats 4 days ago. I acclimated by floating the bags of fish and then adding my tank water to the store water, then netting the fish and adding to tank. I have had almost all the new fish die except the cory cats who seem to be doing well (My penguin tetra died which is a total bummer, had him for years). The new fish look to have swim bladder issues and float vertically, then die. I feed only enough that they eat with none left floating.

Water testing yesterday yielded: Nitrite 0, pH 7.6 and ammonia .25. I did a 25% water change yesterday and changed the carbon in the filter and ammonia is now 0.

I know the ammonia was high, but would that cause the swim bladder like issues? I am at a loss. I have one of the red fin tetras in a quarantine with some methylene blue because I just don't know what else to do...he is floating around on his side and back.

I have included a picture of the tank, there is no undergravel filter and I do weekly water changes.

Can anyone offer suggestions?