Did I read correctly that you are doing half gallon water changes? Your water changes should be more like 50%. I would do a 50% water change once each week and continue 30% to 50% water changes weekly as your regular aquarium maintenance. For the fish in your tank I would keep the tank at a minimum of 78 degrees. I keep my tank at 82, it gets warmer in the summer months too and the fish are fine. I agree with Saqib, fish can survive a lot. I would not be to quick to
euthanize your angel.
Angels plecos and cories don't like salt. If it is swim bladder thawed frozen peas work well.