Since all four fish are in separate tanks and they all developed issues immediately after the last water change, it sounds like the problem occurred with the water change.
I would recommend doing 100% water changes in all of the tanks. Hopefully, this will correct any issue that occurred with the last change.
To do the water change and acclimate them slowly:
- Note the temperature of the water.
- Using a plastic cup, scoop the fish, along with some of his water, into the cup.
- Clean out the tank. Refill with water at the SAME temp. Be sure to add the correct amount of water conditioner.
- Float his cup in the tank for about 15 min.
- Add a SMALL amount of NEW water to the cup. (Several tablespoons, or about 1.5 ounces.)
- Let his cup float for about 10 min.
- Remove a SMALL amount of water from the cup. Discard it.
- Repeat steps 5-7, until about an hour has passed.
- Gently release him into the tank.